USMLE-STEP-1 Online Practice Questions and Answers

Correct Answer: C

Section: Anatomy Centroacinar cells are components of the exocrine pancreas and they contain zymogen granules, which are released under the action of cholecystokinin from the duodenum. The zymogen granules contain inactive proenzymes that normally become activated within the duodenum. Premature activation of the pancreatic enzymes, such as trypsin, leads to autodigestion of the centroacinar cells of the pancreatic acini, which secrete these enzymes. This results in acute hemorrhagic pancreatitis which can be caused by excessive alcohol ingestion. All the other choices are cells of the endocrine pancreas, which secrete hormones. Alpha cells (choice A) secrete glucagon whereas beta cells (choice B) produce insulin. Delta cells (choice C) synthesize gastrin and somatostatin. F cells produce pancreatic polypeptide.

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.

Correct Answer: A

Section: Physiology Intravenous solutions are distributed in the various body fluid compartments based on osmolality and their ability to penetrate the vascular wall and the cell membrane. The proteins in blood plasma will remain within the vascular compartment because of their low permeability across the vessel wall. Thus, 500 mL of blood plasma will remain in the vascular compartment. Water (choice B) passes across the cell membrane and the vascular wall easily. Hence distilled water will distribute itself between cellular and extracellular spaces, meaning only a small portion will actually contribute to blood volume, the other choices (C, D, and E) cross the vascular barrier and distribute partially to the extravascular space and thus are less effective intravascular volume expanders.

Correct Answer: A

Section: Physiology This patient probably suffers from myopia (nearsightedness). Myopia is either due to eyeballs that are too long or a lens that is too strong. To focus a distant object onto the retina (far accommodation), the lens has to decrease its refractive power (i.e., increase its focal length). This is accomplished through relaxation of the ciliary muscles that oppose the pull of the sclera, resulting in a tightening of the zonula fibers and a flattening of the lens. Shortening of the focal length (choice C), rounding of the lens (choice D), and relaxation of the zonular fibers (choice E), all occur during near accommodation. The pupils also constrict during near accommodation (choice B), perhaps to increase depth of field.

Correct Answer: E

Section: Physiology Hyponatremia of this magnitude for any reason is likely to cause seizures and coma, especially if the deviation from normal occurred rapidly (e.g., over 1 or 2 days). Withdrawal from painful stimuli (choice A) is a normal reaction. If intact, it would have provided information on the intactness of sensation and motor reflexes on the tested side. However, lack of withdrawal would be consistent with either a structural or a metabolic cause of coma, and therefore does not help. Blood in the cerebral spinal fluid (choice B) in a nontraumatic lumbar puncture suggests a structural rather than a metabolic cause of coma. Pupillary light responses (choice C) are typically preserved in metabolic encephalopathies. Posturing of limbs (choice D) is suggestive of a focal process involving the brainstem or midbrain rather than a metabolic encephalopathy.

Correct Answer: C

Section: Physiology Destruction of lung tissue is a hallmark of emphysema. Due to this loss of elastic tissue, lung elastic recoil is decreased. Decreased elastic recoil (not increased, choice E) shifts the mechanical balance point between chest wall and lung to a higher volume--FRC is, therefore, increased. The barrel chest is characteristic of emphysema. Destruction of alveolar walls impairs gas exchange, causing an increase in physiological dead space (choice A). Tissue loss also causes a loss of radial traction with an increase in airway resistance--FVC is decreased (choice D). Emphysema is one type of chronic obstructive pulmonary disease. A decrease in the FEV1/FVC is characteristic of an obstructive disease (choice B).

Correct Answer: C

Section: Biochemistry Acromegaly results when there is an excess production of growth hormone after epiphysial closure and cessation of long bone growth. Excessive production of corticotropin-releasing hormone (choice A) would result in an increase in ACTH production, which would lead to enhanced glucocorticoid and mineralocorticoid production. Excessive production of GnRH (choice B) would lead to increased production of LH, FSH, and hCG, with consequent effects on the female reproductive system. Excessive production of IGF-II (choice D) could lead to abnormal neonatal development since it is expressed only during this period. Excessive production of thyroid-stimulating hormone (choice E) would lead to increased release of the thyroid hormones, leading to multisystemic involvement such as rapid heart rate, nervousness, inability to sleep, weight loss, excessive sweating, and sensitivity to heat.

Correct Answer: A

Section: Microbiology/Immunology Prevention of varicella and disseminated zoster rests on the administration of acyclovir. Killed varicellazoster virus (VZV) vaccine has not been shown to be effective. Furthermore, if the immunological system has been compromised the immune responses will be very weak and of questionable value (choice C). Live attenuated VZV vaccines cannot be given to immunocompromised individuals because vaccination can be lethal (choice D). Subunit VZV vaccines are not available, and will be of little value to persons with suppressed immunological responses (choice E). Indinavir is a protease inhibitor used for the treatment of AIDS, not for varicella or zoster (choice B).

Correct Answer: C

Section: Microbiology/Immunology The enterobacteriaceae are a large, heterogeneous group of gram-negative rods that normally inhabit the intestinal tract of humans and animals. The Escherichia genus (usual flora) can incidentally cause disease while Salmonella and Shigella genera are regularly pathogenic for humans. E. coli is the most common cause of urinary tract infection, but there are also several E. coli-associated diarrheal diseases that occur worldwide. Enterohemorrhagic E. coli (EHEC) produces verotoxin and is associated with hemorrhagic colitis and hemolytic uremic syndrome. E. coli serotype O157:H7 (choice C) is the most common strain for these disease presentations. Many outbreaks could be prevented by thoroughly cooking ground beef. EPEC is a strain that causes diarrhea in infants while ETEC is a common cause of traveler's diarrhea. C. jejuni (choice A) can be responsible for a shigella-like illness that is self- limited. Milk or undercooked fowl are frequent vectors of this organism. Non- 01 serogroup of V. cholerae (choice B) only causes a cholera-like illness occasionally. Salmonella (choice D) enterocolitis can be caused by 1400 serotypes. Nausea, headache, vomiting, and profuse diarrhea with low-grade fever are common and resolve in 23 days. The natural habitat of shigellae (choice E) is limited to the intestinal tract of humans and other primates, where they produce bacillary dysentery.

Correct Answer: C