USMLE USMLE Certifications USMLE-STEP-1 Questions & Answers

• Question 691:

  Patients with functional dyspepsia (disturbed indigestion) and prominent nausea frequently experience spurts of excessive acid exposure to the upper duodenum. This results in pancreatic secretion, mainly through the action of which of the following substances?

  A. cholecystokinin

  B. gastrin

  C. glucagon

  D. secretin

  E. vasoactive intestinal polypeptide (VIP)

  Correct Answer: D

  Section: Physiology The strongest stimulator for the release of secretin from cells in the upper small-intestinal mucosa is the contact with acidic chyme. Increased serum secretin levels stimulate water and alkali secretions from the pancreas and the hepatic ducts and inhibit gastrin release. The release of pancreatic enzymes is stimulated by cholecystokinin (choice A). The most potent stimulators for the release of cholecystokinin are not acid, but digestion products of fat and protein. Strong stimulators for gastrin secretion (choice B) are vagus nerve excitation, distention of the stomach, and protein digestion products. Gastrin then stimulates acid secretion and mucosal growth. The major effect of glucagon (choice C) is to increase blood glucose levels. Hence, it is secreted in response to hypoglycemia and protein digestion products, which are then used for gluconeogenesis. VIP (choice E) indeed stimulates intestinal and pancreatic secretion. However, it acts as neurotransmitter in the enteric nervous system and is mainly released by mechanical and neuronal stimulation.

• Question 692:

  An advertisement promotes energy bars containing fructose as an ideal food to take on extreme mountain-climbing expeditions. Which of the following statements concerning fructose absorption is true?

  A. absorption of fructose into an intestinal epithelial cell is by facilitated transport and thus does not require energy

  B. metabolism of fructose generates more energy than glucose

  C. some fructose is already absorbed in the mouth and hence is the fastest way to get energy

  D. the presence of fructose aids in absorption of vitamin A, C, and D

  E. the presence of fructose inhibits reabsorption of glucose, which is then more readily available for muscle activity

  Correct Answer: A

  Section: Physiology Carbohydrate absorption occurs at enterocytes of the upper region of small intestinal villi. Fructose absorption is via the facilitated transporters GLUT5 across the apical enterocyte membrane and GLUT2 across the basolateral enterocyte. Glucose and galactose on the other hand are transported into enterocytes on carriers in combination with a sodium ion. The energy for this secondary active transport is provided by the electrochemical sodium gradient that is created by Na/K-ATPases. Experimental conditions that collapse the sodium electrochemical gradient, hypoxia, or poisoning of the Na-KATPase by ouabain inhibit glucose, but not fructose absorption. Nevertheless, the physiological importance of "saving energy" under extreme conditions such as mountain climbing through the use of fructose as energy source is questionable. For instance, fructose absorption is much slower than absorption of glucose and galactose. The statements in choices B, C, D, and E are incorrect.

• Question 693:

  A 55-year-old male diabetic has an accommodative power of the lens of 10 dioptres. His near point is located 5 cm (2 in), his far point 10 cm (3.9 in) in front of the eye. Which of the following statements are correct?

  A. his corrective lenses are convex

  B. his corrective lenses have a positive dioptric value

  C. the patient has hyperopia

  D. the patient is capable of driving a car without corrective glasses

  E. the patient is functionally blind

  Correct Answer: E

  Section: Physiology Functionally blind means that a person has a visual impairment, that does not qualify as "legally blind" but results in substantial impediment. With a near point of 5 cm and a far point of 10 cm, the man has a severe case of myopia, not hyperopia as stated in choice C. The total convergence power of the relaxed eye with normal vision is approximately 60 dioptres, and the cornea accounts for more than two-thirds of that (40 dioptres). The accommodative power of the lens is about 20 dioptres in the very young, about 10 dioptres at age 25, and would be around 1 dioptre at the patient's age, if he had normal vision. For young adults with normal vision, the near point is about 10 cm from the eye; the far point is at infinity. The corrective lenses for the myopic eye are concave, not convex (choice A). Concave lenses compensate for the excessive positive dioptres of the myopic eye. These lenses are thin in the middle and wide at the edges and have negative dioptric values, not positive ones (choice B). The patient definitely won't be able to drive a car or perform other activities that require fast accommodation without corrective glasses (choice D).

• Question 694:

  A 54-year-old insulin-dependent diabetic notes that her insulin requirements have gone up dramatically in the past year (from 50 U to nearly 200 U of recombinant human insulin) and her blood glucose is still poorly controlled. A possible explanation for the worsening of her diabetes includes which of the following?

  A. a high titer of anti-insulin antibodies

  B. an improved diet

  C. an improved exercise program

  D. progression of macrovascular disease

  E. weight loss

  Correct Answer: A

  Section: Physiology The patient clearly has an increase in her state of insulin resistance. Given the magnitude of her increased insulin requirements, she most likely developed a high titer of antiinsulin antibodies that are preventing the injected insulin from lowering blood glucose effectively. Agood choice of lifestyle, including an improved diet (choice B), an improved exercise program (choice C), or weight loss (choice E) each are shown to beneficially affect her insulin requirements. Progression of macrovascular disease (choice D) is largely irrelevant to her insulin requirements, except to the extent that it might decrease her ability to exercise.

• Question 695:

  below figure shows a lead II electrocardiogram from a normal 30-year-old female at rest. If a betaadrenergic agonist such as isoproterenol is administered intravenously to this woman, what will be her resulting heart rate in beats per minute?

  

  A. 40

  B. 55

  C. 65

  D. 70

  E. 90

  Correct Answer: E

  Section: Physiology From inspection of the figure, the R-R interval (time between heart beats) at rest is 0.83 seconds; 60 seconds/minute divided by 0.83 yields a resting heart rate of 72 beats/minute. Since activation of receptors in the sinoatrial node will result in increased heart rate, the only alternative larger than 72 beats/ minute is E., 90 beats/minute.

• Question 696:

  With increasing age, it is quite common for adults to have very low sensitivity for highpitch sounds. Which of the following physiologic responses occur as the pitch of a sound is increased at constant sound pressure?

  A. a greater number of hair cells become activated

  B. the amplitude of maximal basilar membrane displacement increases

  C. the frequency of action potentials in auditory nerve fibers increases

  D. the location of maximal basilar membrane displacement moves toward the cochlear base

  E. units in the auditory nerve become responsive to a wider range of sound frequencies

  Correct Answer: D

  Section: Physiology The primary change in the cochlea due to an increase in the frequency of a sound wave is a change in the position of maximal displacement of the basilar membrane. A low-pitched sound produces the greatest displacement toward the apex of the cochlea, which results in activation of hair cells at that location. As pitch increases, the position of greatest displacement moves closer to the base of the cochlea. Increases in the number of hair cells that are activated (choice A), increased amplitude of basilar membrane displacement (choice B), increases in the frequency of discharge of units in the auditory nerve fibers (choice C), and an increase in range of frequencies to which such units respond (choice E), are all more likely to be observed in response to increases in the intensity of the sound stimulus (sound pressure) rather than to increases in pitch.

• Question 697:

  A 20-year-old woman admits herself to the emergency room with a yellow discoloration of the whites of her eyes. She says that she does not drink and that she has not experienced any changes in her stool. Her liver enzyme profile and direct serum bilirubin levels are normal, while total bilirubin is elevated. What is the most likely cause for her jaundice?

  A. defect in hepatocytes

  B. defect in Kupffer cells

  C. gallstones

  D. hemolysis

  E. tumor obstructing bile duct

  Correct Answer: D

  Section: Physiology Rupture of large numbers of red blood cells can result in jaundice in the absence of any liver disease. The capacity of the liver to clear released heme metabolites such as bilirubin is temporarily exceeded. Since the liver will not perform its normal function to conjugate bilirubin before excretion in bile and urine, unconjugated bilirubin backs up in serum. This is the reason for the increase in total bilirubin, while conjugated bilirubin, also called direct bilirubin, is not affected. Direct bilirubin would be elevated with most defects of liver cells (choice A) and with obstruction of bile flow by gallstones (choice C) or by a tumor (choice E). All these would most likely also affect bile formation and consequently absorption of dietary fat and fat-soluble vitamins, with the consequence of developing oily, fatty stool. Kupffer cells (choice B) are macrophages within the liver and don't play a role in the production of bilirubin.

• Question 698:

  Pupil size is an important indicator of brainstem function. Which of the following results in papillary constriction?

  A. atropine, a blocker of muscarinic receptors

  B. decreased parasympathetic activity of inner eye muscle fibers during darkness

  C. general increased sympathetic tone during emotional excitement

  D. increased sympathetic activity of inner eye muscle fibers during darkness

  E. phentolamine, a blocker of alpha adrenergic receptors

  Correct Answer: E

  Section: Physiology Pupil diameter is determined by the balance between sympathetic tone to the radial fibers of the iris and parasympathetic tone to the circular pupillary sphincter muscle. Sympathetic activation will result in pupillary dilation via activation of alpha-adrenergic receptors. Hence, an alpha-adrenergic blocker leads to constriction of the pupil. Parasympathetic activation on the other hand will result in pupil constriction via activation of muscarinic acetylcholine receptors, so that pupil dilation occurs as a result of a muscarinic acetylcholine receptor block (choice A). The same is true for decreased parasympathetic activity during darkness (choice B), or increased sympathetic activity, independent if the initial stimulus is due to emotional excitement or darkness (choices C and D).

• Question 699:

  For the bithermal caloric test, a measure of vestibular sensitivity, warm or cold water is introduced into the ear. This often causes vertigo and nausea as a side effect as a result of which of the following?

  A. convection currents in endolymph

  B. especially fast flow of endolymph

  C. temporary immobilization of cristae ampullares

  D. temporary immobilization of otolith organs

  E. temporary inhibition of cochlear hair cells

  Correct Answer: A

  Section: Physiology Water that is either warmer or cooler than body temperature when introduced into the external auditory meatus sets up convection currents within the endolymph of the inner ear. These currents may result in the movement of the cupula, the gelatinous matrix, which sits atop the crista ampullaris and occludes the lumen of the semicircular canal. Displacement of the cupula results in activation of the vestibular hair cells of the ampullar crista. This inappropriate stimulation of the vestibular system conflicts with sensory information from other sensor organs, including vestibular sensors on the contralateral side, vision, and proprioception, and may in turn result in vertigo and nausea. The existence of the many ducts within the inner ear might imply that the fluids that fill them are flowing through the ear. However, neither endolymph nor perilymph "flow" along their respective compartments in the normal or stimulated vestibular system (choice B). Neither hair cells of cristae ampullares (choice C) nor of otholith organs (choice D) nor of the cochlea (choice E) are temporarily immobilized or inhibited by temperature changes.

• Question 700:

  The ventricular action potential labeled N in below figure represents a normal ventricular action potential. Which of the other action potentials (labeled AE) would be most characteristic of the ventricular cell after administration of a drug that partially blocks slow calcium channels?

  

  A. A

  B. B

  C. C

  D. D

  E. E

  Correct Answer: D

  Section: Physiology The slow calcium channels function in the ventricular myocardial cell to allow the influx of calcium into the cell, which has the dual role of triggering the calcium-mediated contractile mechanism and maintaining the depolarized state after the voltage-gated sodium channels close, soon after the rapid upstroke. Hence, blocking these channels will shorten the plateau phase of the action potential as seen in choice D, while also decreasing the contractile force of the heart. In choice A action potential duration is also shortened, but in a manner that would reflect blockade of the rapid voltagegated sodium channels, not the slow calcium channels. Choice B represents no change in the action potential. Choice C would occur if the drug prolonged the open state of the slow calcium channels. The hyperpolarization of choice E is more likely the result of increased potassium channel currents.