Medical Tests Medical Tests Certifications MCAT-TEST Questions & Answers

• Question 491:

  Oversecretion of gastric HCl can be treated by severing the vagus nerve in a procedure called a vagotomy which reduces parasympathetic activity. Which of the following effects is LEAST likely to be caused by a vagotomy?

  A. Decreased heart rate

  B. Decreased gastric motility

  C. Decreased HCl production

  D. Increased blood pressure

  Correct Answer: A

  The parasympathetic nervous system is involved in "rest-and-digest" functions which include lowering the heart rate and promoting digestion. The question stem states that a vagotomy will reduce parasympathetic stimulation. This would tend

  to increase heart rate and reduce gastrointestinal activity. The question asks for the LEAST likely effect of a vagotomy. A vagotomy would not decrease heart rate. Choices B and C are incorrect because decreased parasympathetic activity

  would decrease gastric motility and HCl production.

  Choice D is incorrect because decreased parasympathetic activity would increase heart rate, and thus increase blood pressure.

• Question 492:

  Tetrodotoxin, the extremely potent poison produced by the puffer (fugu) fish, binds tightly to Na+ channels and blocks the flow of Na+ ions but does not affect K+ or Cl- channels. Tetrodotoxin directly blocks which phase of action potential propagation?

  A. Depolarization

  B. Repolarization

  C. Hyperpolarization

  D. Saltatory conduction

  Correct Answer: A

  An action potential begins with the opening of voltage gated sodium channels that allow sodium to flow into the axon, depolarizing the cell. This is followed by opening of potassium channels that allow potassium to flow out of the cell,

  repolarizing it. Blocking the sodium channels would block depolarization. Choice B is incorrect because repolarization is achieved primarily by opening potassium channels. Choice C is incorrect because hyperpolarization is also due primarily

  to the flux of potassium ions out of the axon.

  Choice D is incorrect because saltatory conduction refers to conduction between nodes of Ranvier in myelinated fibers.

• Question 493:

  Female athletes may fail to menstruate (amenorrhea) because extremely low body fat reduces the release of GnRH from the hypothalamus, affecting the secretion of LH and FSH. Which of the following is most likely to be absent in a female long-distance runner?

  A. Ovary

  B. Anterior pituitary

  C. Corpus luteum

  D. Hypothalamus

  Correct Answer: C

  The question stem states that female athletes may not menstruate and discusses a decrease in LH and FSH secretion. It can be inferred that ovulation does not occur. The corpus luteum is formed, following ovulation, by the cells of the follicle. Without ovulation, there will be no formation of the corpus luteum. Choices A, B, and C are incorrect because lack of any of these structures could not be caused by excercise. The anterior pituitary and hypothalamus are structures in the brain which serve endocrine roles. The ovary is the female gonad, located in the abdomen, which is responsible for egg production and release.

• Question 494:

  Artificial kidneys have been used for almost 50 years to treat patients with different forms of renal failure. The artificial kidney (dialysis machine) removes unwanted substances from the blood by diffusion. A patient's blood is passed through channels bounded by a porous, semi-permeable membrane that allows the free diffusion in both directions of all plasma constituents except the plasma proteins. Erythrocytes and other cellular components of blood cannot pass through the membrane. The other side of the membrane is exposed to the dialyzing fluid which carries away the unwanted materials. If the concentration of a material in the blood is greater than in the dialyzing fluid, there will be a net flow of the material from the plasma to the dialyzing fluid. If the concentration of a material in the blood is less than in the dialyzing fluid, there will be a net flow of the material from the dialyzing fluid into the blood. The composition of normal plasma, plasma in an individual suffering renal failure, and dialyzing fluid are shown in Table 1.

  

  Table 1

  Dialysis replaces some functions of the kidneys and attempts to correct the effects of renal failure. For example, patients with renal failure develop acidosis due to a buildup of metabolically produced acids in the circulation. Without dialysis, the pH of the blood will drop and coma may occur. Dialyzing fluid contains a relatively high concentration of bicarbonate which diffuses into the circulation and neutralizes the acid.

  Healthy kidneys secrete the hormone renin in response to decreased arterial pressure. Renin secretion leads to aldosterone secretion by the adrenal cortex. The body would most likely respond to ingestion of a large volume of isotonic solution by:

  A. decreasing renin secretion and increasing sodium and water reabsorption.

  B. decreasing renin secretion and decreasing sodium and water reabsorption.

  C. increasing renin secretion and increasing sodium and water reabsorption.

  D. increasing renin secretion and decreasing sodium and water reabsorption.

  Correct Answer: B

  The question stem states that kidneys secrete renin in response to decreased arterial pressure. Ingesting a large quantity of isotonic solution would tend to increase the arterial pressure. This would lead to a decrease in renin production and a consequent decrease in aldosterone secretion. Decreased aldosterone secretion would decrease sodium reabsorption which is accompanied by reabsorption of water. Decreasing reabsorption of water tends to increase water excretion and reduce blood pressure. This is shown in the flow chart below:

  

  Choice A is incorrect because sodium reabsorption would decrease. Choice C is incorrect because renin production would decrease. Choice D is incorrect because renin production would decrease.

• Question 495:

  Artificial kidneys have been used for almost 50 years to treat patients with different forms of renal failure. The artificial kidney (dialysis machine) removes unwanted substances from the blood by diffusion. A patient's blood is passed through channels bounded by a porous, semi-permeable membrane that allows the free diffusion in both directions of all plasma constituents except the plasma proteins. Erythrocytes and other cellular components of blood cannot pass through the membrane. The other side of the membrane is exposed to the dialyzing fluid which carries away the unwanted materials. If the concentration of a material in the blood is greater than in the dialyzing fluid, there will be a net flow of the material from the plasma to the dialyzing fluid. If the concentration of a material in the blood is less than in the dialyzing fluid, there will be a net flow of the material from the dialyzing fluid into the blood. The composition of normal plasma, plasma in an individual suffering renal failure, and dialyzing fluid are shown in Table 1.

  

  Table 1

  Dialysis replaces some functions of the kidneys and attempts to correct the effects of renal failure. For example, patients with renal failure develop acidosis due to a buildup of metabolically produced acids in the circulation. Without dialysis, the pH of the blood will drop and coma may occur. Dialyzing fluid contains a relatively high concentration of bicarbonate which diffuses into the circulation and neutralizes the acid.

  All of the following are removed from the plasma by dialysis EXCEPT:

  A. Na+

  B. K+

  C. Urea

  D. Glucose

  Correct Answer: D

  By comparing the concentration of a material in the dialyzing fluid with the concentration in plasma, we can determine the net flow by passive diffusion through the semi-permeable membrane. Of all of the choices, only glucose has a higher

  concentration in the dialyzing fluid than in the plasma (with or without renal failure). Thus, glucose will not be removed from the plasma by dialysis. Choice A is incorrect because the concentration of Na+ in the dialyzing fluid is 133 mEq/L

  which is less than the concentration in plasma (142 mEq/L). Sodium will be removed.

  Choice B is incorrect because the concentration of K+ (1.0 mEq/L) is much lower than the concentration in the plasma.

  Choice C is incorrect because there is no urea in the dialyzing fluid. One of the primary functions of dialysis is the removal of urea from the circulation.

• Question 496:

  Artificial kidneys have been used for almost 50 years to treat patients with different forms of renal failure. The artificial kidney (dialysis machine) removes unwanted substances from the blood by diffusion. A patient's blood is passed through channels bounded by a porous, semi-permeable membrane that allows the free diffusion in both directions of all plasma constituents except the plasma proteins. Erythrocytes and other cellular components of blood cannot pass through the membrane. The other side of the membrane is exposed to the dialyzing fluid which carries away the unwanted materials. If the concentration of a material in the blood is greater than in the dialyzing fluid, there will be a net flow of the material from the plasma to the dialyzing fluid. If the concentration of a material in the blood is less than in the dialyzing fluid, there will be a net flow of the material from the dialyzing fluid into the blood. The composition of normal plasma, plasma in an individual suffering renal failure, and dialyzing fluid are shown in Table 1.

  

  Table 1

  Dialysis replaces some functions of the kidneys and attempts to correct the effects of renal failure. For example, patients with renal failure develop acidosis due to a buildup of metabolically produced acids in the circulation. Without dialysis, the pH of the blood will drop and coma may occur. Dialyzing fluid contains a relatively high concentration of bicarbonate which diffuses into the circulation and neutralizes the acid.

  A patient with renal failure has nephrons which lack the ability to actively secrete or reabsorb any substances. Which of the following actions will the patient's kidney still be able to perform?

  A. Removal of salt from the blood

  B. Production of hypertonic urine

  C. Production of hypotonic urine

  D. Conservation of amino acids

  Correct Answer: A

  Salt is removed from the blood by filtration at the glomerulus. Recall that the fluid in Bowman's capsule is isotonic to plasma. Without reabsorption or secretion, isotonic urine can still be produced. Choice B is incorrect because production of hypertonic urine requires the concentration gradient between the medulla and cortex of the kidney. This gradient is generated by active secretion and reabsorption. Choice C is incorrect because production of hypotonic urine requires active reabsorption and secretion. Choice D is incorrect because amino acids are filtered and then actively reabsorbed in the proximal convoluted tubule.

• Question 497:

  Artificial kidneys have been used for almost 50 years to treat patients with different forms of renal failure. The artificial kidney (dialysis machine) removes unwanted substances from the blood by diffusion. A patient's blood is passed through channels bounded by a porous, semi-permeable membrane that allows the free diffusion in both directions of all plasma constituents except the plasma proteins. Erythrocytes and other cellular components of blood cannot pass through the membrane. The other side of the membrane is exposed to the dialyzing fluid which carries away the unwanted materials. If the concentration of a material in the blood is greater than in the dialyzing fluid, there will be a net flow of the material from the plasma to the dialyzing fluid. If the concentration of a material in the blood is less than in the dialyzing fluid, there will be a net flow of the material from the dialyzing fluid into the blood. The composition of normal plasma, plasma in an individual suffering renal failure, and dialyzing fluid are shown in Table 1.

  

  Table 1

  Dialysis replaces some functions of the kidneys and attempts to correct the effects of renal failure. For example, patients with renal failure develop acidosis due to a buildup of metabolically produced acids in the circulation. Without dialysis,

  the pH of the blood will drop and coma may occur. Dialyzing fluid contains a relatively high concentration of bicarbonate which diffuses into the circulation and neutralizes the acid.

  Which of the following provides the best explanation for the urea plasma concentration in individuals with renal failure?

  A. Urea filtration decreases

  B. Urea absorption decreases

  C. Urea filtration increases

  D. Urea secretion increases

  Correct Answer: A

  Reading from Table 1, it is clear that the urea concentration in a patient with renal failure is much higher than that in a normal individual. Filtration of urea decreases, leading to decreased excretion and a higher urea concentration in the

  plasma.

  Choice B is incorrect because a decrease of absorption from the nephron tubule would tend to increase excretion and decrease the plasma urea concentration.

  Choice C is incorrect because increased urea filtration would lead to increased excretion. Choice D is incorrect because increased secretion of urea into the nephron tubule would increase excretion.

• Question 498:

  Artificial kidneys have been used for almost 50 years to treat patients with different forms of renal failure. The artificial kidney (dialysis machine) removes unwanted substances from the blood by diffusion. A patient's blood is passed through channels bounded by a porous, semi-permeable membrane that allows the free diffusion in both directions of all plasma constituents except the plasma proteins. Erythrocytes and other cellular components of blood cannot pass through the membrane. The other side of the membrane is exposed to the dialyzing fluid which carries away the unwanted materials. If the concentration of a material in the blood is greater than in the dialyzing fluid, there will be a net flow of the material from the plasma to the dialyzing fluid. If the concentration of a material in the blood is less than in the dialyzing fluid, there will be a net flow of the material from the dialyzing fluid into the blood. The composition of normal plasma, plasma in an individual suffering renal failure, and dialyzing fluid are shown in Table 1.

  

  Table 1

  Dialysis replaces some functions of the kidneys and attempts to correct the effects of renal failure. For example, patients with renal failure develop acidosis due to a buildup of metabolically produced acids in the circulation. Without dialysis,

  the pH of the blood will drop and coma may occur. Dialyzing fluid contains a relatively high concentration of bicarbonate which diffuses into the circulation and neutralizes the acid.

  The semi-permeable membrane of the dialysis machine functions in a manner most analogous to which part of the kidney?

  A. Glomerulus

  B. Ureter

  C. Descending loop of Henle endothelium

  D. Vasa recta

  Correct Answer: A

  The glomerulus functions like a sieve, allowing the filtration (movement from the circulation into the nephron tubule) of small molecules while blocking the filtration of the plasma proteins. The semi-permeable membrane serves an analogous function in the dialysis machine. Choice B is incorrect because the ureter is merely a tube connecting the kidney to the bladder. Choice C is incorrect because the descending loop of Henle endothelium does not serve a filtration function. The primary action of the descending loop is reabsorption of water. Choice D is incorrect because the vasa recta are the capillaries that supply nutrients to the nephron.

• Question 499:

  Artificial kidneys have been used for almost 50 years to treat patients with different forms of renal failure. The artificial kidney (dialysis machine) removes unwanted substances from the blood by diffusion. A patient's blood is passed through channels bounded by a porous, semi-permeable membrane that allows the free diffusion in both directions of all plasma constituents except the plasma proteins. Erythrocytes and other cellular components of blood cannot pass through the membrane. The other side of the membrane is exposed to the dialyzing fluid which carries away the unwanted materials. If the concentration of a material in the blood is greater than in the dialyzing fluid, there will be a net flow of the material from the plasma to the dialyzing fluid. If the concentration of a material in the blood is less than in the dialyzing fluid, there will be a net flow of the material from the dialyzing fluid into the blood. The composition of normal plasma, plasma in an individual suffering renal failure, and dialyzing fluid are shown in Table 1.

  

  Table 1

  Dialysis replaces some functions of the kidneys and attempts to correct the effects of renal failure. For example, patients with renal failure develop acidosis due to a buildup of metabolically produced acids in the circulation. Without dialysis,

  the pH of the blood will drop and coma may occur. Dialyzing fluid contains a relatively high concentration of bicarbonate which diffuses into the circulation and neutralizes the acid.

  In order to prevent the net movement of water between the blood and the dialyzing fluid, the dialyzing fluid:

  A. is hypoosmotic to blood.

  B. is isoosmotic to blood.

  C. contains a higher concentration of solutes than blood.

  D. contains hydrophilic proteins.

  Correct Answer: B

  If the dialyzing fluid is isoosmotic, it has the same concentration of particles and thus, the same osmotic pressure exists on either side of the membrane. There will be no net flow of water by osmosis between the blood and the dialyzing fluid. Choice A is incorrect because a hypoosmotic dialyzing fluid would lead to flow of water into the circulation from the dialyzing fluid. Choice C is incorrect because a solution with a higher concentration of solutes is hyperosmotic. A hyperosmotic dialyzing fluid would lead to flow of water out of the circulation from the dialyzing fluid. Choice D is incorrect because hydrophilicity has nothing to do with the net flow of water. A hydrophilic protein is "water-loving" because it contains polar amino acids.

• Question 500:

  The hydrogens of alkanes have pKa values that are over 30 or 40. In contrast, the -hydrogens of aldehydes and ketones have pKa values that range from 19 to 21. These fairly acidic -hydrogens can be removed by strong bases to form anions called enolates. The enolate ions are strongly stabilized by resonance. Protonation of the enolate at oxygen produces an enol. Interconversion between the keto and enol forms is called tautomerization and is illustrated in Figure 1. The keto form is usually highly favored.

  

  Figure 1

  Keto-enol tautomerization has some interesting consequences. For example, if a ketone is treated with acid or base in a solvent of D2O (heavy water), all of the - hydrogens will be exchanged for deuterium. This reaction is shown in Figure 2.

  

  Figure 2 Another consequence of keto-enol tautomerization is the racemization of chiral -carbons. In the enol form, the -carbon adopts a planar configuration and is no longer chiral. Tautomerization back to the ketone produces a racemic mixture of products. This is shown in Figure 3.

  

  Figure 3

  Which of the following best represents the enolate ion intermediate of the racemization reaction shown in Figure 3?

  

  A. Option A

  B. Option B

  C. Option C

  D. Option D

  Correct Answer: C

  The enolate anion is stabilized by resonance which delocalizes the negative charge to the electronegative oxygen.

  Choice A is incorrect because it shows delocalization of a positive charge. The enolate ion is a negative ion (anion) created by loss of an alpha proton as described in the passage. Choice B is incorrect because, although this enolate anion

  may form through abstraction of an alpha proton from the methyl group, it will not lead to racemization of the chiral center. Note that the structure in Choice B maintains a chiral center which has not been turned into an achiral, planar

  configuration which would lead to racemization. Choice D is incorrect because this resonance structure would not occur.