Medical Tests Medical Tests Certifications MCAT-TEST Questions & Answers

• Question 521:

  In the small intestine, most amino acids are transported from the lumen of the gut into the epithelial cells against their concentration gradient. This movement of amino acids is coupled to the movement of sodium ions down their concentration gradient in a form of transmembrane movement called:

  A. simple diffusion.

  B. facilitated diffusion.

  C. secondary active transport.

  D. primary active transport.

  Correct Answer: C

  Secondary active transport is defined as the transport of a material across the cell membrane against its concentration gradient without direct hydrolysis of ATP. The energy required to drive this active transport is acquired by harnessing favorable concentrations gradients, like the flow of sodium ion into a cell. Note that the sodium gradient is created by the primary active transport of sodium out of the cell by the sodium/ potassium pump. Choice A is incorrect because simple diffusion is the movement of materials down a concentration gradient through the membrane without the assistance of pumps or protein channels. Small molecules () and non-polar molecules (steroids) can move into a cell by simple diffusion. Choice B is incorrect because facilitated diffusion is movement of a material along its concentration gradient through protein channels. Ions can move in and out of the cell through specific ion channels. Choice D is incorrect because primary active transport involves transport against a concentration gradient driven directly by ATP hydrolysis. The sodium/potassium pump is an ATPase (hydrolyzes ATP).

• Question 522:

  Hypoxia refers to a physiological condition in which the body lacks sufficient oxygen for normal cellular functioning. Prolonged hypoxia generally leads to an inhibition of mental capacity and a reduction in the work capacity of muscle. Severe cases of hypoxia can lead to coma or even death. Depending on the cause, hypoxia can be classified into four general types:

  Hypoxic hypoxia is a type of hypoxia that occurs when the partial pressure of oxygen in the blood is too low. For example, climbers at high altitude, where the air contains less oxygen, might experience hypoxic hypoxia because the partial pressure of oxygen in the air inhaled is very low, leading to insufficient partial pressure of oxygen in the blood.

  Anemic hypoxia describes a diminished ability of the blood to transport oxygen. Several factors can influence the oxygen-carrying capacity of the blood. Primary causes of anemic hypoxia include a lower than normal number of functional erythrocytes or an insufficient quantity of hemoglobin, the oxygen- carrying molecules of the blood. Abnormal hemoglobin can also decrease the blood's capacity to carry oxygen and lead to anemic hypoxia.

  Ischemic hypoxia is caused by a decreased delivery of blood to the tissues. Localized circulatory deficiencies, such as blood clots, and global circulatory deficiencies, such as heart failure, decrease the delivery of blood to the tissues, and can therefore cause ischemic hypoxia.

  Histotoxic hypoxia results from the inability of cells to utilize the oxygen available in the blood. Causes of histotoxic hypoxia include the poisoning of cellular enzymes involved in aerobic respiration, as well as the decreased metabolic capacity of the oxidative enzymes due to vitamin deficiency. Cyanide poisoning causes histotoxic hypoxia by blocking the action of cytochrome oxidase in the electron transport chain so that tissues cannot use oxygen even though it is available.

  A patent foramen ovale occurs when an infant's foramen ovale does not close completely at birth. Based on the information presented in the passage, this can lead to:

  A. hypoxic hypoxia because much of the blood does not reach the lungs to be oxygenated leading to a low

  B. anemic hypoxia because the blood does not carry enough oxygen.

  C. ischemic hypoxia because the flow of blood is shunted away from the lungs.

  D. none of the above.

  Correct Answer: A

  The foramen ovale is a shunt that diverts blood away from the lungs in the fetal circulatory route. The foramen ovale allows blood to enter the left atrium directly from the right atrium. If an infant's foramen ovale does not close completely at birth, deoxygenated blood will continue to flow from the right atrium directly to the left atrium without ever being oxygenated in the lungs. In the left atrium, the deoxygenated blood will mix with the oxygenated blood returning from the lungs. The overall effect will be hypoxic hypoxia as the PO2 of the blood flowing to the body is decreased. Choice B is incorrect because the defining characteristic of anemic hypoxia is the reduction of the oxygen- carrying capacity of the blood. In this case, the carrying capacity of the blood has not changed (there is no anemia or damage to hemoglobin). The problem is that much of the infant's blood never reaches the lungs to be oxygenated. The infant's blood is capable of carrying oxygen, it just does not receive enough oxygen to carry. Choice C is incorrect because the blood flow is not shunted away from the tissues which need oxygen. In ischemic hypoxia, the movement of blood is reduced, thus reducing delivery of oxygen to the tissues. In this case, the flow of blood throughout the body is not affected. It is true that the flow of blood through the heart and lungs is not normal, but the flow of blood to the tissues is normal. Choice D is incorrect because Choice A is correct.

• Question 523:

  Hypoxia refers to a physiological condition in which the body lacks sufficient oxygen for normal cellular functioning. Prolonged hypoxia generally leads to an inhibition of mental capacity and a reduction in the work capacity of muscle. Severe

  cases of hypoxia can lead to coma or even death. Depending on the cause, hypoxia can be classified into four general types:

  Hypoxic hypoxia is a type of hypoxia that occurs when the partial pressure of oxygen in the blood is too low. For example, climbers at high altitude, where the air contains less oxygen, might experience hypoxic hypoxia because the partial pressure of oxygen in the air inhaled is very low, leading to insufficient partial pressure of oxygen in the blood.

  Anemic hypoxia describes a diminished ability of the blood to transport oxygen. Several factors can influence the oxygen-carrying capacity of the blood. Primary causes of anemic hypoxia include a lower than normal number of functional erythrocytes or an insufficient quantity of hemoglobin, the oxygen- carrying molecules of the blood. Abnormal hemoglobin can also decrease the blood's capacity to carry oxygen and lead to anemic hypoxia.

  Ischemic hypoxia is caused by a decreased delivery of blood to the tissues. Localized circulatory deficiencies, such as blood clots, and global circulatory deficiencies, such as heart failure, decrease the delivery of blood to the tissues, and can therefore cause ischemic hypoxia.

  Histotoxic hypoxia results from the inability of cells to utilize the oxygen available in the blood. Causes of histotoxic hypoxia include the poisoning of cellular enzymes involved in aerobic respiration, as well as the decreased metabolic capacity of the oxidative enzymes due to vitamin deficiency. Cyanide poisoning causes histotoxic hypoxia by blocking the action of cytochrome oxidase in the electron transport chain so that tissues cannot use oxygen even though it is available.

  Hypoxia can often be treated by ventilation with pure oxygen. The increased PO2 in the alveoli will lead to an increased PO2 in the blood. Treatment with pure oxygen is LEAST effective in treating which of the following types of hypoxia?

  A. Hypoxic hypoxia caused by hypoventilation

  B. Anemic hypoxia caused by sickle cell anemia

  C. Ischemic hypoxia caused by poor circulation

  D. Histotoxic hypoxia caused by the disease beri beri

  Correct Answer: D

  Oxygen therapy will help any condition which would be alleviated by an increase in the p of oxygen in the blood. The question stem states that beri beri leads to histotoxic hypoxia. The passage tells us that histoxic hypoxia occurs when the tissues cannot utilize oxygen. So, increasing the amount of oxygen transported in the blood through ventilation with pure oxygen will not serve any purpose. Choice A is incorrect because breathing pure oxygen will greatly help an individual suffering from hypoventilation. If pure oxygen is inhaled, much more oxygen will diffuse into the blood and be carried to the tissues. Choice B is incorrect because, although sickle cell anemia reduces the oxygen carrying capacity of the blood (anemic hypoxia), inhaling pure oxygen will allow more oxygen to dissolve in the plasma, somewhat increasing delivery of oxygen to the tissues. Choice C is incorrect because increasing the concentration of oxygen in the blood will increase delivery of oxygen to the tissues and take full advantage of the carrying capacity of the poor circulation.

• Question 524:

  Hypoxia refers to a physiological condition in which the body lacks sufficient oxygen for normal cellular functioning. Prolonged hypoxia generally leads to an inhibition of mental capacity and a reduction in the work capacity of muscle. Severe cases of hypoxia can lead to coma or even death. Depending on the cause, hypoxia can be classified into four general types:

  Hypoxic hypoxia is a type of hypoxia that occurs when the partial pressure of oxygen in the blood is too low. For example, climbers at high altitude, where the air contains less oxygen, might experience hypoxic hypoxia because the partial pressure of oxygen in the air inhaled is very low, leading to insufficient partial pressure of oxygen in the blood.

  Anemic hypoxia describes a diminished ability of the blood to transport oxygen. Several factors can influence the oxygen-carrying capacity of the blood. Primary causes of anemic hypoxia include a lower than normal number of functional erythrocytes or an insufficient quantity of hemoglobin, the oxygen- carrying molecules of the blood. Abnormal hemoglobin can also decrease the blood's capacity to carry oxygen and lead to anemic hypoxia.

  Ischemic hypoxia is caused by a decreased delivery of blood to the tissues. Localized circulatory deficiencies, such as blood clots, and global circulatory deficiencies, such as heart failure, decrease the delivery of blood to the tissues, and can therefore cause ischemic hypoxia.

  Histotoxic hypoxia results from the inability of cells to utilize the oxygen available in the blood. Causes of histotoxic hypoxia include the poisoning of cellular enzymes involved in aerobic respiration, as well as the decreased metabolic capacity of the oxidative enzymes due to vitamin deficiency. Cyanide poisoning causes histotoxic hypoxia by blocking the action of cytochrome oxidase in the electron transport chain so that tissues cannot use oxygen even though it is available.

  The passages of the respiratory tract which do not participate in gas exchange are called the physiological dead space. Compared to air in the alveoli, air in the physiological dead space will have:

  

  A. Option A

  B. Option B

  C. Option C

  D. Option D

  Correct Answer: C

  The question stem states that the physiological dead space is not involved in gas exchange. This means that the composition of air in the dead space is virtually identical to that of atomospheric air. In the alveoli, where gas exchange occurs, oxygen is taken up by the blood and carbon dioxide is released. Thus, the air in the alveoli will have a lower pO2 and a higher pCO2 than dead space air. Conversely, dead space air will have a higher pO2 and a lower pCO2 than alveolar air. Choices A, B, and D are incorrect because they do not indicate that the alveolar air will have a higher concentration of carbon dioxide and a lower concentration of oxygen.

• Question 525:

  Hypoxia refers to a physiological condition in which the body lacks sufficient oxygen for normal cellular functioning. Prolonged hypoxia generally leads to an inhibition of mental capacity and a reduction in the work capacity of muscle. Severe cases of hypoxia can lead to coma or even death. Depending on the cause, hypoxia can be classified into four general types:

  Hypoxic hypoxia is a type of hypoxia that occurs when the partial pressure of oxygen in the blood is too low. For example, climbers at high altitude, where the air contains less oxygen, might experience hypoxic hypoxia because the partial pressure of oxygen in the air inhaled is very low, leading to insufficient partial pressure of oxygen in the blood.

  Anemic hypoxia describes a diminished ability of the blood to transport oxygen. Several factors can influence the oxygen-carrying capacity of the blood. Primary causes of anemic hypoxia include a lower than normal number of functional erythrocytes or an insufficient quantity of hemoglobin, the oxygen- carrying molecules of the blood. Abnormal hemoglobin can also decrease the blood's capacity to carry oxygen and lead to anemic hypoxia.

  Ischemic hypoxia is caused by a decreased delivery of blood to the tissues. Localized circulatory deficiencies, such as blood clots, and global circulatory deficiencies, such as heart failure, decrease the delivery of blood to the tissues, and can therefore cause ischemic hypoxia.

  Histotoxic hypoxia results from the inability of cells to utilize the oxygen available in the blood. Causes of histotoxic hypoxia include the poisoning of cellular enzymes involved in aerobic respiration, as well as the decreased metabolic capacity of the oxidative enzymes due to vitamin deficiency. Cyanide poisoning causes histotoxic hypoxia by blocking the action of cytochrome oxidase in the electron transport chain so that tissues cannot use oxygen even though it is available.

  Carbon monoxide binds to hemoglobin as shown in the dissociation curve below. The dissociation curve for oxygen is also shown. These dissociation curves indicate that:

  

  A. carbon monoxide binds to hemoglobin with the same affinity as does oxygen.

  B. carbon monoxide saturates hemoglobin at much lower partial pressures than does oxygen.

  C. oxygen will saturate hemoglobin more rapidly than will an equal partial pressure of carbon monoxide.

  D. carbon monoxide forms covalent bonds to hemoglobin while oxygen is held by non-covalent interactions.

  Correct Answer: B

  Although the shape of the curves is similar, the X-axis indicates that hemoglobin becomes saturated with CO at a much lower partial pressure (0.4 mmHg) than with oxygen (140 mmHg). Choice A is incorrect because the graphs indicate that carbon monoxide binds with a higher affinity. Choice C is incorrect because oxygen will not saturate hemoglobin as much (or as rapidly) as an equal partial pressure of carbon monoxide. Choice D is incorrect because the structure of the binding of oxygen and carbon monoxide to hemoglobin cannot be determined from the graphs.

• Question 526:

  Hypoxia refers to a physiological condition in which the body lacks sufficient oxygen for normal cellular functioning. Prolonged hypoxia generally leads to an inhibition of mental capacity and a reduction in the work capacity of muscle. Severe cases of hypoxia can lead to coma or even death. Depending on the cause, hypoxia can be classified into four general types:

  Hypoxic hypoxia is a type of hypoxia that occurs when the partial pressure of oxygen in the blood is too low. For example, climbers at high altitude, where the air contains less oxygen, might experience hypoxic hypoxia because the partial pressure of oxygen in the air inhaled is very low, leading to insufficient partial pressure of oxygen in the blood.

  Anemic hypoxia describes a diminished ability of the blood to transport oxygen. Several factors can influence the oxygen-carrying capacity of the blood. Primary causes of anemic hypoxia include a lower than normal number of functional erythrocytes or an insufficient quantity of hemoglobin, the oxygen- carrying molecules of the blood. Abnormal hemoglobin can also decrease the blood's capacity to carry oxygen and lead to anemic hypoxia.

  Ischemic hypoxia is caused by a decreased delivery of blood to the tissues. Localized circulatory deficiencies, such as blood clots, and global circulatory deficiencies, such as heart failure, decrease the delivery of blood to the tissues, and can therefore cause ischemic hypoxia.

  Histotoxic hypoxia results from the inability of cells to utilize the oxygen available in the blood. Causes of histotoxic hypoxia include the poisoning of cellular enzymes involved in aerobic respiration, as well as the decreased metabolic capacity of the oxidative enzymes due to vitamin deficiency. Cyanide poisoning causes histotoxic hypoxia by blocking the action of cytochrome oxidase in the electron transport chain so that tissues cannot use oxygen even though it is available.

  If cyanide is radioactively-labeled and its position traced within the cell, it will most likely be found in:

  A. the Golgi apparatus.

  B. membrane bound vesicles.

  C. the nucleus.

  D. the mitochondria.

  Correct Answer: D

  The passage states that cyanide blocks the action of cytochrome oxidase in the electron transport chain. The enzymes of the electron transport chain, including cytochrome oxidase, are located on the inner membrane of the mitochondria. If

  cyanide blocks the action of cytochrome oxidase, it is most likely located in the same place.

  Choices A, B, and C are incorrect because these cellular locations are not involved in the electron transport chain which is blocked by cyanide action.

• Question 527:

  Hypoxia refers to a physiological condition in which the body lacks sufficient oxygen for normal cellular functioning. Prolonged hypoxia generally leads to an inhibition of mental capacity and a reduction in the work capacity of muscle. Severe cases of hypoxia can lead to coma or even death. Depending on the cause, hypoxia can be classified into four general types:

  Hypoxic hypoxia is a type of hypoxia that occurs when the partial pressure of oxygen in the blood is too low. For example, climbers at high altitude, where the air contains less oxygen, might experience hypoxic hypoxia because the partial pressure of oxygen in the air inhaled is very low, leading to insufficient partial pressure of oxygen in the blood.

  Anemic hypoxia describes a diminished ability of the blood to transport oxygen. Several factors can influence the oxygen-carrying capacity of the blood. Primary causes of anemic hypoxia include a lower than normal number of functional erythrocytes or an insufficient quantity of hemoglobin, the oxygen- carrying molecules of the blood. Abnormal hemoglobin can also decrease the blood's capacity to carry oxygen and lead to anemic hypoxia.

  Ischemic hypoxia is caused by a decreased delivery of blood to the tissues. Localized circulatory deficiencies, such as blood clots, and global circulatory deficiencies, such as heart failure, decrease the delivery of blood to the tissues, and can therefore cause ischemic hypoxia.

  Histotoxic hypoxia results from the inability of cells to utilize the oxygen available in the blood. Causes of histotoxic hypoxia include the poisoning of cellular enzymes involved in aerobic respiration, as well as the decreased metabolic capacity of the oxidative enzymes due to vitamin deficiency. Cyanide poisoning causes histotoxic hypoxia by blocking the action of cytochrome oxidase in the electron transport chain so that tissues cannot use oxygen even though it is available.

  Cigarette smoking causes emphysema, a condition in which the net surface area of the lungs is greatly decreased leading to a decrease in the diffusing capacity of the lungs.

  Emphysema leads to which type of hypoxia?

  A. Hypoxic hypoxia

  B. Anemic hypoxia

  C. Ischemic hypoxia

  D. Histotoxic hypoxia

  Correct Answer: A

  The passage states that hypoxic hypoxia is caused by any factor that leads to a decreased plasma pO2. Since emphysema decreases the diffusing capacity of the lungs, less oxygen will diffuse into the blood, leading to a lower pO2. Choice B is incorrect because anemic hypoxia is caused by a decreased oxygen carrying capacity of the blood. Emphysema affects the amount of oxygen in the blood, not the ability of the blood to carry oxygen. Choice C is incorrect because ischemic hypoxia is caused by a decreased delivery of blood to the tissues. Emphysema has no effect on blood circulation. Choice D is incorrect because histotoxic hypoxia is caused by the inability of the tissues to utilize oxygen. Emphysema does not affect the ability of the tissues to use oxygen.

• Question 528:

  Hypoxia refers to a physiological condition in which the body lacks sufficient oxygen for normal cellular functioning. Prolonged hypoxia generally leads to an inhibition of mental capacity and a reduction in the work capacity of muscle. Severe cases of hypoxia can lead to coma or even death. Depending on the cause, hypoxia can be classified into four general types:

  Hypoxic hypoxia is a type of hypoxia that occurs when the partial pressure of oxygen in the blood is too low. For example, climbers at high altitude, where the air contains less oxygen, might experience hypoxic hypoxia because the partial pressure of oxygen in the air inhaled is very low, leading to insufficient partial pressure of oxygen in the blood.

  Anemic hypoxia describes a diminished ability of the blood to transport oxygen. Several factors can influence the oxygen-carrying capacity of the blood. Primary causes of anemic hypoxia include a lower than normal number of functional erythrocytes or an insufficient quantity of hemoglobin, the oxygen- carrying molecules of the blood. Abnormal hemoglobin can also decrease the blood's capacity to carry oxygen and lead to anemic hypoxia.

  Ischemic hypoxia is caused by a decreased delivery of blood to the tissues. Localized circulatory deficiencies, such as blood clots, and global circulatory deficiencies, such as heart failure, decrease the delivery of blood to the tissues, and can therefore cause ischemic hypoxia.

  Histotoxic hypoxia results from the inability of cells to utilize the oxygen available in the blood. Causes of histotoxic hypoxia include the poisoning of cellular enzymes involved in aerobic respiration, as well as the decreased metabolic capacity of the oxidative enzymes due to vitamin deficiency. Cyanide poisoning causes histotoxic hypoxia by blocking the action of cytochrome oxidase in the electron transport chain so that tissues cannot use oxygen even though it is available.

  Exposure to high levels of radiation has been demonstrated to cause anemia. The most likely explanation is that radiation damages the:

  A. blood vessels.

  B. spleen.

  C. thymus.

  D. bone marrow.

  Correct Answer: D

  All blood cells (RBCs and WBCs) are produced by division of stem cells in the bone marrow. Damage to the bone marrow would therefore affect the formation of blood cells which would be likely to result in decreased production of erythrocytes (RBCs). Since the passage states that anemia is defined as a decrease in the number of functional erythrocytes, damage to the bone marrow by exposure to radiation could be a cause of anemia. Choice A is incorrect because damage to the blood vessels could affect blood circulation but would not affect the number of erythrocytes in the blood. Choices B and C are incorrect because the spleen and thymus are involved in the development and maturation of lymphocytes, a type of white blood cells, and not of red blood cells. Damage to the spleen or thymus, which would affect the number of lymphocytes, would not necessarily affect the number of red blood cells.

• Question 529:

  Synthetic dyes constitute a commercially significant area of organic chemistry. The color producing properties of these compounds are the result of highly delocalized electron systems giving rise to electronic transitions whose absorptions occur in the visible region. Most commercially useful dyes can be classified as one of three types -- anthraquinones, azo dyes, or triarylmethyl salts. Examples of each type are illustrated in Figure 1.

  

  Figure 1

  In order for a dye to be useful in the fabric industry, it must have sufficient affinity for the polymeric fibers of which the material is composed; the dye must not only impart a color to the fabric, but must also do so in a relatively permanent manner (color fastness). Proper design of synthetic polymers requires the placement of acidic or basic side chains along the polymer backbone such that binding sites are available for dying. Similarly, dyes must be produced not only with the appropriate color-producing structure, but also with an affinity for the fabric in question. The structural units of several common synthetic fibers are shown in Figure 2.

  

  Figure 2

  Hydrogenation of alizarin over a specific rhodium catalyst results in the complete saturation of all the carbon-carbon bonds, but does not affect any other functional groups. Which of the following effects should also be expected?

  A. The hydrogenated product will no longer have any affinity for Dacron fibers.

  B. The hydrogenated product will no longer be soluble in aqueous base.

  C. The hydrogenated product will no longer appear colored.

  D. The hydrogenated product will have a greatly increased affinity for nylon 610.

  Correct Answer: C

  As stated in the passage, color is the result of electronic transitions due to the delocalized system in the dye. If the two aromatic rings of alizarin are completely hydrogenated, the resulting compound will contain only the two isolated ketone double bonds; the system will thus no longer be delocalized, and color production will cease. Choice A is incorrect since the two H groups would still be present in the hydrogenated product; it is this feature of the molecule which accounts for the affinity for Dacron, via hydrogen bonding. Choice B is wrong since solubility in aqueous base is mainly due to the presence of the two H groups, and to a lesser extent to the polar carbonyl bonds, which will still be present in the hydrogenated product. Choice D is incorrect because it is not the aromatic nature of alizarin which accounts for its affinity, or lack thereof, for nylon 66. As the passage states, side chains are often necessary to provide a binding site; this is very likely to be the case between nylon and alizarin.

• Question 530:

  Synthetic dyes constitute a commercially significant area of organic chemistry. The color producing properties of these compounds are the result of highly delocalized electron systems giving rise to electronic transitions whose absorptions occur in the visible region. Most commercially useful dyes can be classified as one of three types -- anthraquinones, azo dyes, or triarylmethyl salts. Examples of each type are illustrated in Figure 1.

  

  Figure 1

  In order for a dye to be useful in the fabric industry, it must have sufficient affinity for the polymeric fibers of which the material is composed; the dye must not only impart a color to the fabric, but must also do so in a relatively permanent manner (color fastness). Proper design of synthetic polymers requires the placement of acidic or basic side chains along the polymer backbone such that binding sites are available for dying. Similarly, dyes must be produced not only with the appropriate color-producing structure, but also with an affinity for the fabric in question. The structural units of several common synthetic fibers are shown in Figure 2.

  

  Figure 2

  Azo dyes are synthesized via diazotization. The sequence of diazotization involves first treating an aromatic amine with NaNO2 in acidic solution, thereby converting it to the corresponding diazonium salt (with the general formula ArN2X-), then reacting with a second aromatic compound to form the azo dye. What is the structure of the azo dye produced by the reaction sequence below?

  

  A. Option A

  B. Option B

  C. Option C

  D. Option D

  Correct Answer: D

  Aromatic diazonium salts have the general formula ArN2+X-. The second step of the diazotization reaction described is electrophilic aromatic substitution, with ArN2+ acting as the electrophile, thus one should predict that the nitrogen of the salt should attach to the second ring in the position dictated by the second ring's substituents. Since the second aromatic compound, p-nitroanisole, has two substituents, we need to examine the directing effect of each: the nitro group is a meta director, while the ether is an ortho/para director. The effects of these two groups should thus be additive, placing the new substituent ortho to the CH3, as shown in choice D. Choices A and B are incorrect since the substitution reaction will take place at one of the ring protons rather than one of the substituent groups. Choice C is wrong because O2 is meta directing but is forced here to be ortho directing.