Medical Tests Medical Tests Certifications MCAT-TEST Questions & Answers

• Question 601:

  Electromagnetic radiation from space constantly bombards the earth. Most wavelengths are absorbed by the atmosphere; however, there are two "windows" of nonabsorption through which significant amounts of radiation reach the ground. The first transmits ultraviolet and visible light, as well as infrared light or heat; the second transmits radio waves. As a result, terrestrial organisms have evolved a number of pigments that interact with light in various ways: some capture light energy, some provide protection from light- induced damage, and some serve camouflage or signaling purposes.

  Among these compounds are many conjugated polyenes, which play important roles as photoreceptors. For every chemical compound, there are certain wavelengths of light whose quanta possess exactly the correct amount of energy to raise electrons from their ground state to higher-energy orbitals. For most organic compounds, these wavelengths are in the UV range. However, conjugated double bond systems stabilize the electrons, so that they can be excited by lower-frequency photons with wavelengths in the visible spectrum. Such a pigment, known as a chromophore, will then transmit the "subtraction color," a color complementary to the one absorbed. For instance, carotene, a hydrocarbon compound with eleven conjugated double bonds, absorbs blue light and transmits orange. The wavelength that is absorbed generally increases with the number of conjugated bonds; rings and side-chains also affect wavelength.

  Wavelength Color Subtraction Color 480 nm blue orange 580 nm yellow violet 680 nm red green

  Among the many biological molecules that are affected by light is DNA, the genetic material of living organisms. DNA absorbs ultraviolet light, and may be damaged by UVC (< 280 nm) and UVB (280-315 nm). UVA (315-400 nm) and visible light can actually repair light-induced damage to DNA by a process called photorepair. For this reason, UVA, which also stimulates tanning, was once considered beneficial. However, there is now increasing evidence that UVA can damage skin.

  Many crustaceans produce a blue or green caroteneprotein complex. What is the most likely cause of the color change from green to orange when a lobster is boiled?

  A. Heat causes the prosthetic group to become partially hydrated.

  B. The increase in temperature permits the prosthetic group to absorb shorter wavelengths.

  C. The protein is separated from the carotenoid pigment.

  D. Heat causes the prosthetic group to become oxidized.

  Correct Answer: C

  The green color comes from absorption of light by the entire carotene-protein complex; as you know from the passage, the carotene by itself is orange. The fact that boiled lobsters are red and not orange is due to other pigments that are also present. Anyway, when a lobster is cooked, the heat disrupts the intermolecular attractions that attach the carotene molecule, the protein's prosthetic group, to the protein itself. This change in the molecule causes the wavelength of light absorbed to change as well. As a matter of fact, it reduces the number of conjugated double bonds and therefore shortens the wavelength of light absorbed. The red color of the cooked lobster is the transmission color of the isolated carotenoid, with the conjugated bonds from the protein removed. Choices A and D are wrong because if carotene were hydrated or oxidized, it would break the chain of conjugation completely and produce a colorless compound. An increase in temperature, choice B, is only a physical change and won't change the absorption wavelength.

• Question 602:

  Electromagnetic radiation from space constantly bombards the earth. Most wavelengths are absorbed by the atmosphere; however, there are two "windows" of nonabsorption through which significant amounts of radiation reach the ground.

  The first transmits ultraviolet and visible light, as well as infrared light or heat; the second transmits radio waves. As a result, terrestrial organisms have evolved a number of pigments that interact with light in various ways: some capture light

  energy, some provide protection from light- induced damage, and some serve camouflage or signaling purposes.

  Among these compounds are many conjugated polyenes, which play important roles as photoreceptors. For every chemical compound, there are certain wavelengths of light whose quanta possess exactly the correct amount of energy to

  raise electrons from their ground state to higher-energy orbitals. For most organic compounds, these wavelengths are in the UV range. However, conjugated double bond systems stabilize the electrons, so that they can be excited by lower-

  frequency photons with wavelengths in the visible spectrum. Such a pigment, known as a chromophore, will then transmit the "subtraction color," a color complementary to the one absorbed. For instance, carotene, a hydrocarbon compound

  with eleven conjugated double bonds, absorbs blue light and transmits orange. The wavelength that is absorbed generally increases with the number of conjugated bonds; rings and side-chains also affect wavelength.

  Wavelength Color Subtraction Color

  480 nm blue orange

  580 nm yellow violet

  680 nm red green

  Among the many biological molecules that are affected by light is DNA, the genetic material of living organisms. DNA absorbs ultraviolet light, and may be damaged by UVC (< 280 nm) and UVB (280-315 nm). UVA (315-400 nm) and visible

  light can actually repair light-induced damage to DNA by a process called photorepair. For this reason, UVA, which also stimulates tanning, was once considered beneficial. However, there is now increasing evidence that UVA can damage

  skin.

  Why is benzene colorless?

  A. The absorption energy is of too high a frequency to be visible.

  B. The absorption energy is of too low a frequency to be visible.

  C. Benzene does not absorb light.

  D. Benzene is not conjugated.

  Correct Answer: A

  The greater the number of conjugated bonds in a molecule, the more strongly electrons in excited states will be stabilized. Thus the more conjugated a molecule, the lower the frequency and the longer the wavelength that will excite it. Benzene, with only three double bonds, requires absorption of quite high energy light, higher than for the colored compounds discussed in the passage. In fact, the light required to excite benzene electrons is in the ultraviolet range, so no color is produced when benzene absorbs light, and benzene is colorless.

• Question 603:

  Electromagnetic radiation from space constantly bombards the earth. Most wavelengths are absorbed by the atmosphere; however, there are two "windows" of nonabsorption through which significant amounts of radiation reach the ground. The first transmits ultraviolet and visible light, as well as infrared light or heat; the second transmits radio waves. As a result, terrestrial organisms have evolved a number of pigments that interact with light in various ways: some capture light energy, some provide protection from light- induced damage, and some serve camouflage or signaling purposes.

  Among these compounds are many conjugated polyenes, which play important roles as photoreceptors. For every chemical compound, there are certain wavelengths of light whose quanta possess exactly the correct amount of energy to raise electrons from their ground state to higher-energy orbitals. For most organic compounds, these wavelengths are in the UV range. However, conjugated double bond systems stabilize the electrons, so that they can be excited by lower-frequency photons with wavelengths in the visible spectrum. Such a pigment, known as a chromophore, will then transmit the "subtraction color," a color complementary to the one absorbed. For instance, carotene, a hydrocarbon compound with eleven conjugated double bonds, absorbs blue light and transmits orange. The wavelength that is absorbed generally increases with the number of conjugated bonds; rings and side-chains also affect wavelength.

  Wavelength Color Subtraction Color 480 nm blue orange 580 nm yellow violet 680 nm red green Among the many biological molecules that are affected by light is DNA, the genetic material of living organisms. DNA absorbs ultraviolet light, and may be damaged by UVC (< 280 nm) and UVB (280-315 nm). UVA (315-400 nm) and visible light can actually repair light-induced damage to DNA by a process called photorepair. For this reason, UVA, which also stimulates tanning, was once considered beneficial. However, there is now increasing evidence that UVA can damage skin.

  Two pigments are identical except for the lengths of their conjugated polyene chains. The first transmits yellow light and the second red. What can be said about the sizes of the chromophores?

  A. The first is longer.

  B. The second is longer.

  C. One of the chromophores must be a dimer.

  D. The comparative lengths cannot be determined.

  Correct Answer: B

  For each compound, the light that's transmitted is complementary in color to the light absorbed; thus, the first chromophore must absorb violet light and the second must absorb green light. Violet light has a shorter wavelength than green, meaning it has a higher energy. You're told that longer chromophores generally absorb longer-wavelength light than shorter ones, so the one that absorbs the green light, and therefore has a red color, must be the longer one. This makes B correct and A and D incorrect. Even though you know one pigment is longer, there's no basis for concluding that it is a dimer (meaning a compound made up of two identical subunits), so choice C is definitely wrong.

• Question 604:

  Electromagnetic radiation from space constantly bombards the earth. Most wavelengths are absorbed by the atmosphere; however, there are two "windows" of nonabsorption through which significant amounts of radiation reach the ground. The first transmits ultraviolet and visible light, as well as infrared light or heat; the second transmits radio waves. As a result, terrestrial organisms have evolved a number of pigments that interact with light in various ways: some capture light energy, some provide protection from light- induced damage, and some serve camouflage or signaling purposes.

  Among these compounds are many conjugated polyenes, which play important roles as photoreceptors. For every chemical compound, there are certain wavelengths of light whose quanta possess exactly the correct amount of energy to raise electrons from their ground state to higher-energy orbitals. For most organic compounds, these wavelengths are in the UV range. However, conjugated double bond systems stabilize the electrons, so that they can be excited by lower-frequency photons with wavelengths in the visible spectrum. Such a pigment, known as a chromophore, will then transmit the "subtraction color," a color complementary to the one absorbed. For instance, carotene, a hydrocarbon compound with eleven conjugated double bonds, absorbs blue light and transmits orange. The wavelength that is absorbed generally increases with the number of conjugated bonds; rings and side-chains also affect wavelength.

  Wavelength Color Subtraction Color 480 nm blue orange 580 nm yellow violet 680 nm red green

  Among the many biological molecules that are affected by light is DNA, the genetic material of living organisms. DNA absorbs ultraviolet light, and may be damaged by UVC (< 280 nm) and UVB (280-315 nm). UVA (315-400 nm) and visible light can actually repair light-induced damage to DNA by a process called photorepair. For this reason, UVA, which also stimulates tanning, was once considered beneficial. However, there is now increasing evidence that UVA can damage skin.

  The electrons that give color to a carotene molecule are found in:

  A. s orbitals.

  B. orbitals.

  C. d orbitals.

  D. f orbitals.

  Correct Answer: B

  Double bonds, like the ones in carotene, consist of one sigma bond, formed by the overlap of two sp2 orbitals, and one bond, formed by the overlap of perpendicular p orbitals. It is the latter, the bond, that is stabilized by the conjugated polyene system. This conjugation makes the electrons able to be excited by lower frequencies of light. Choice A, s orbitals, is wrong because the electrons which give conjugated bonds their stability, are not in s orbitals. The other two choices are wrong because carbon and hydrogen, which are the main or only components of organic molecules like carotene, don't have d and f orbitals.

• Question 605:

  Just as the ingestion of nutrients is mandatory for human life, so is the excretion of metabolic waste products. One of these nutrients, protein, is used for building muscle, nucleic acids, and countless compounds integral to homeostasis. However, the catabolism of the amino acids generated from protein digestion produces ammonia, which, if not further degraded, can become toxic. Similarly, if the same salts that provide energy and chemical balance to cells are in excess, fluid retention will occur, damaging the circulatory, cardiac, and pulmonary systems.

  One of the most important homeostatic organs is the kidney, which closely regulates the excretion and reabsorption of many essential ions and molecules. One mechanism of renal function involves the secretion of antidiuretic hormone (ADH).

  Diabetes insipidus (DI), is the condition that occurs when ADH is ineffective. As a result, the kidneys are unable to concentrate urine, leading to excessive water loss. There are two types of DI -- central and nephrogenic. Central DI occurs when there is a deficiency in the quantity or quality of ADH produced. Nephrogenic DI occurs when the kidney tubules are unresponsive to ADH. To differentiate between these two conditions, a patient's urine osmolarity is measured both prior to therapy and after a 24-hour restriction on fluid intake. Exogenous ADH is then administered and urine osmolarity is measured again. The table below gives the results of testing on four patients. Assume that a urine osmolarity of 285 mOsm/L of H2O is normal.

  

  What is the most likely cause of Patient B's dilute urine before therapy?

  A. Excessive water intake

  B. Dehydration

  C. Nephrogenic DI

  D. Central DI

  Correct Answer: A

  Fluid restriction is the first step in the attempt to diagnose the pathology, if there is one, behind the inability to concentrate urine. Let's take a look at the results of therapy on Patient B. Patient B had a low urine osmolarity prior to the onset of therapy. However, following the restriction of fluid intake, there was a substantial increase in urine osmolarity, indicating that Patient B's dilute urine is a function of the amount of fluid ingested. If a lot of fluid is drunk during the day, then assuming the person's kidneys and ADH are both normal, the urine formed will be dilute. Likewise, if there is very little fluid drunk during the day, then the urine formed will be concentrated. Thus, the excessive intake of water seems to be the most likely explanation for the formation of dilute urine in Patient B. In fact, there are individuals who have a psychological condition in which they drink water excessively. This excessive intake causes the output of dilute urine. Simply restricting and observing fluid intake would enable the patient to concentrate urine. Fluid restriction had no effect on either Patient C or Patient D, indicating that they have a true pathology; so choices C and D are wrong. If Patient B was dehydrated, then the body's response would be to produce concentrated urine from the start, which was clearly not the case here since urine osmolarity was very low prior to therapy; therefore, choice B is also wrong.

• Question 606:

  Just as the ingestion of nutrients is mandatory for human life, so is the excretion of metabolic waste products. One of these nutrients, protein, is used for building muscle, nucleic acids, and countless compounds integral to homeostasis. However, the catabolism of the amino acids generated from protein digestion produces ammonia, which, if not further degraded, can become toxic. Similarly, if the same salts that provide energy and chemical balance to cells are in excess, fluid retention will occur, damaging the circulatory, cardiac, and pulmonary systems.

  One of the most important homeostatic organs is the kidney, which closely regulates the excretion and reabsorption of many essential ions and molecules. One mechanism of renal function involves the secretion of antidiuretic hormone (ADH).

  Diabetes insipidus (DI), is the condition that occurs when ADH is ineffective. As a result, the kidneys are unable to concentrate urine, leading to excessive water loss. There are two types of DI -- central and nephrogenic. Central DI occurs when there is a deficiency in the quantity or quality of ADH produced. Nephrogenic DI occurs when the kidney tubules are unresponsive to ADH. To differentiate between these two conditions, a patient's urine osmolarity is measured both prior to therapy and after a 24-hour restriction on fluid intake. Exogenous ADH is then administered and urine osmolarity is measured again. The table below gives the results of testing on four patients. Assume that a urine osmolarity of 285 mOsm/L of H2O is normal.

  

  Based on the data in Table 1, which of the four patients most likely has nephrogenic diabetes insipidus?

  A. Patient A

  B. Patient B

  C. Patient C

  D. Patient D

  Correct Answer: C

  You're told that a normal value for urine osmolarity is 285 milli-osmoles per liter of H2O. Since ADH increases water reabsorption in the kidneys, patients with diabetes insipidus are expected to have a decreased urine osmolarity. And if normal is 285 milli-osmoles per liter of water, then based on the information in Table 1, you should have concluded that Patient A does not have diabetes insipidus. Patients B, C, and D, have a very low urine osmolarity prior to therapy, indicating that there is something wrong. To answer this question, you must have a good understanding of the mechanisms behind both central and nephrogenic diabetes insipidus. According to the passage, central diabetes insipidus is when ADH itself is either deficient in quantity or quality. Therefore, exogenous supplementation of ADH SHOULD alleviate the symptoms; that is, the kidneys should be able to concentrate urine, and therefore, urine osmolarity should greatly increase only AFTER ADH is administered. It should NOT increase after the 24- hour restriction of fluid intake because water is not being reabsorbed.

• Question 607:

  Just as the ingestion of nutrients is mandatory for human life, so is the excretion of metabolic waste products. One of these nutrients, protein, is used for building muscle, nucleic acids, and countless compounds integral to homeostasis. However, the catabolism of the amino acids generated from protein digestion produces ammonia, which, if not further degraded, can become toxic. Similarly, if the same salts that provide energy and chemical balance to cells are in excess, fluid retention will occur, damaging the circulatory, cardiac, and pulmonary systems.

  One of the most important homeostatic organs is the kidney, which closely regulates the excretion and reabsorption of many essential ions and molecules. One mechanism of renal function involves the secretion of antidiuretic hormone (ADH).

  Diabetes insipidus (DI), is the condition that occurs when ADH is ineffective. As a result, the kidneys are unable to concentrate urine, leading to excessive water loss. There are two types of DI -- central and nephrogenic. Central DI occurs when there is a deficiency in the quantity or quality of ADH produced. Nephrogenic DI occurs when the kidney tubules are unresponsive to ADH. To differentiate between these two conditions, a patient's urine osmolarity is measured both prior to therapy and after a 24-hour restriction on fluid intake. Exogenous ADH is then administered and urine osmolarity is measured again. The table below gives the results of testing on four patients. Assume that a urine osmolarity of 285 mOsm/L of H2O is normal.

  

  Based on the data in Table 1, which of the four patients most likely has central diabetes insipidus?

  A. Patient A

  B. Patient B

  C. Patient C

  D. Patient D

  Correct Answer: C

  You're told that a normal value for urine osmolarity is 285 milli-osmoles per liter of H2O. Since ADH increases water reabsorption in the kidneys, patients with diabetes insipidus are expected to have a decreased urine osmolarity. And if normal is 285 milli-osmoles per liter of water, then based on the information in Table 1, you should have concluded that Patient A does not have diabetes insipidus. Patients B, C, and D, have a very low urine osmolarity prior to therapy, indicating that there is something wrong. To answer this question, you must have a good understanding of the mechanisms behind both central and nephrogenic diabetes insipidus. According to the passage, central diabetes insipidus is when ADH itself is either deficient in quantity or quality. Therefore, exogenous supplementation of ADH SHOULD alleviate the symptoms; that is, the kidneys should be able to concentrate urine, and therefore, urine osmolarity should greatly increase only AFTER ADH is administered. It should NOT increase after the 24- hour restriction of fluid intake because water is not being reabsorbed. If you look at the results of the four patients, you'll see only Patient C's urine osmolarity increased after ADH was administered. So, choice C is the right answer. In the case of nephrogenic diabetes insipidus it is the tubules themselves that are defective. Exogenous ADH would be ineffective as therapy since the patient's own ADH is sufficient in quality and quantity.

• Question 608:

  Just as the ingestion of nutrients is mandatory for human life, so is the excretion of metabolic waste products. One of these nutrients, protein, is used for building muscle, nucleic acids, and countless compounds integral to homeostasis. However, the catabolism of the amino acids generated from protein digestion produces ammonia, which, if not further degraded, can become toxic. Similarly, if the same salts that provide energy and chemical balance to cells are in excess, fluid retention will occur, damaging the circulatory, cardiac, and pulmonary systems.

  One of the most important homeostatic organs is the kidney, which closely regulates the excretion and reabsorption of many essential ions and molecules. One mechanism of renal function involves the secretion of antidiuretic hormone (ADH).

  Diabetes insipidus (DI), is the condition that occurs when ADH is ineffective. As a result, the kidneys are unable to concentrate urine, leading to excessive water loss. There are two types of DI -- central and nephrogenic. Central DI occurs when there is a deficiency in the quantity or quality of ADH produced. Nephrogenic DI occurs when the kidney tubules are unresponsive to ADH. To differentiate between these two conditions, a patient's urine osmolarity is measured both prior to therapy and after a 24-hour restriction on fluid intake. Exogenous ADH is then administered and urine osmolarity is measured again. The table below gives the results of testing on four patients. Assume that a urine osmolarity of 285 mOsm/L of H2O is normal.

  

  Which of the following would you most likely expect to find in a patient with diabetes insipidus?

  A. Decreased plasma osmolarity

  B. Increased urine osmolarity

  C. Increased urine glucose

  D. Increased urine output

  Correct Answer: D

  As explained in the passage, diabetes insipidus is a condition in which ADH is ineffective, and as a result, the kidneys reabsorb less water and are unable to concentrate urine. What is the result of this? Very dilute urine. So, urine output is increased in a patient with diabetes insipidus, which means that choice D is correct. Since water reabsorption is decreased, this means that plasma osmolarity will be increased, and so choice A is wrong. Choice C is wrong because the presence of glucose in the urine is one of the symptoms and tell-tale signs of diabetes mellitus. Diabetes mellitus is the result of an insulin deficiency; without insulin, glucose is not converted into glycogen, and as a result, some of it gets excreted in the urine because the kidneys are overwhelmed by the excess glucose in the bloodstream. Choice B is wrong because an increased urine osmolarity would mean that there was a greater concentration of dissolved solutes per liter of urine, which is not the case if there is an excess of water being excreted. It should be noted that a diabetes insipidus patient with an intact thirst mechanism and access to water will NOT become dehydrated. They will drink enough water to replace what is lost in the urine.

• Question 609:

  Just as the ingestion of nutrients is mandatory for human life, so is the excretion of metabolic waste products. One of these nutrients, protein, is used for building muscle, nucleic acids, and countless compounds integral to homeostasis. However, the catabolism of the amino acids generated from protein digestion produces ammonia, which, if not further degraded, can become toxic. Similarly, if the same salts that provide energy and chemical balance to cells are in excess, fluid retention will occur, damaging the circulatory, cardiac, and pulmonary systems.

  One of the most important homeostatic organs is the kidney, which closely regulates the excretion and reabsorption of many essential ions and molecules. One mechanism of renal function involves the secretion of antidiuretic hormone (ADH).

  Diabetes insipidus (DI), is the condition that occurs when ADH is ineffective. As a result, the kidneys are unable to concentrate urine, leading to excessive water loss. There are two types of DI -- central and nephrogenic. Central DI occurs when there is a deficiency in the quantity or quality of ADH produced. Nephrogenic DI occurs when the kidney tubules are unresponsive to ADH. To differentiate between these two conditions, a patient's urine osmolarity is measured both prior to therapy and after a 24-hour restriction on fluid intake. Exogenous ADH is then administered and urine osmolarity is measured again. The table below gives the results of testing on four patients. Assume that a urine osmolarity of 285 mOsm/L of H2O is normal.

  

  Which of the following substances would NOT be found in appreciable quantity in the urine of a healthy individual?

  A. Albumin

  B. Sodium

  C. Urea

  D. Potassium

  Correct Answer: A

  Albumin is a protein produced by the liver. It is very important in maintaining the plasma osmolarity of the blood. The glomerulus, which is the network of blood vessels enveloped by the part of the nephron known as Bowman's capsule, has small holes in its endothelial lining called fenestrations. When proteins travel through the glomerulus, they are prevented from entering the nephron due to the size of the fenestrations and the negative electrical charge of glycosylated proteins lining the fenestrations. Therefore, albumin is not normally found in urine because it is prevented from entering the nephron, and so choice A is the right answer. The presence of protein in a urine sample is typically a sign of renal disease. Each of the other choices are normally found in appreciable quantity in the urine. Urea, choice C, is the primary excretory byproduct of the urea cycle. Sodium and potassium homeostasis are kept in balance by the regulation of their excretion in urine.

• Question 610:

  Just as the ingestion of nutrients is mandatory for human life, so is the excretion of metabolic waste products. One of these nutrients, protein, is used for building muscle, nucleic acids, and countless compounds integral to homeostasis. However, the catabolism of the amino acids generated from protein digestion produces ammonia, which, if not further degraded, can become toxic. Similarly, if the same salts that provide energy and chemical balance to cells are in excess, fluid retention will occur, damaging the circulatory, cardiac, and pulmonary systems.

  One of the most important homeostatic organs is the kidney, which closely regulates the excretion and reabsorption of many essential ions and molecules. One mechanism of renal function involves the secretion of antidiuretic hormone (ADH).

  Diabetes insipidus (DI), is the condition that occurs when ADH is ineffective. As a result, the kidneys are unable to concentrate urine, leading to excessive water loss. There are two types of DI -- central and nephrogenic. Central DI occurs when there is a deficiency in the quantity or quality of ADH produced. Nephrogenic DI occurs when the kidney tubules are unresponsive to ADH. To differentiate between these two conditions, a patient's urine osmolarity is measured both prior to therapy and after a 24-hour restriction on fluid intake. Exogenous ADH is then administered and urine osmolarity is measured again. The table below gives the results of testing on four patients. Assume that a urine osmolarity of 285 mOsm/L of H2O is normal.

  

  According to the passage, the catabolism of amino acids produces ammonia. Therefore, after a proteinrich meal, would you expect a build-up of ammonia in the lumen of the small intestine?

  A. Yes, because the ammonia will not be able to diffuse into the intestinal epithelium.

  B. Yes, because the rate at which digestive enzymes degrade ammonia is slower than the rate at which ammonia is produced.

  C. No, because the ammonia will diffuse into the intestinal epithelium and will be excreted by the kidneys.

  D. No, because the ammonia is produced inside individual cells, not within the lumen of the small intestine.

  Correct Answer: D

  To answer this question, you need to recall the site of protein digestion, and the fate of the amino acids produced by this digestion. So let's review the digestive system as it relates to proteins. Food enters the stomach from the pharynx and esophagus. When protein-containing food reaches the stomach, its presence causes the release of the hormone gastrin, which stimulates the secretion of HCl, and pepsinogen, and muscular contractions of the stomach. HCl initiates the conversion of pepsinogen to its active form, pepsin. Pepsin breaks down proteins into peptides. The peptides then enter the small intestine. Three types of peptide-digesting enzymes are secreted into the small intestine; enterokinase, aminopeptidases, and dipeptidases. The combination of these three enzyme types breaks apart all peptide bonds, producing only single amino acids and dipeptides. The single amino acids and peptides are then absorbed into the epithelial cells lining the small intestine by active transport. These molecules all enter the bloodstream by way of the capillaries of the villi. Villi are the finger-like projections lining the small intestine that serve to increase the absorptive surface area of the intestine. From the bloodstream the amino acids enter individual cells. Inside these cells, the amino acids are used to either build new proteins or other biological molecules, or they are completely catabolized, in which case ammonia is produced. This ammonia then enters the urea cycle, and the urea produced in the process exits the cells and is excreted in the urine. So from this discussion of the digestion of protein, it is clear that ammonia is NOT produced inside the lumen of the small intestine. Therefore, choices A, B, and C are all wrong and choice D is the correct answer.