USMLE-STEP-1 Online Practice Questions and Answers

Correct Answer: A

Section: Physiology The action potentials illustrated must be those of cardiac nodal cells (SA node or AV node). The duration of these action potentials is too long for motor axons, (choice B), sensory neurons (choice C), or skeletal muscle fibers (choice D). Also, the configuration is different and a pacemaker potential is clearly present. They cannot be ventricular Purkinje action potentials (choice E) because these have a more negative diastolic component that does not gradually depolarize, a longer duration (200 msec) and a plateau region.

Correct Answer: C

Section: Physiology Normally, an elevated plasma osmolarity will stimulate increased ADH secretion and cause increased renal water reabsorption, which should lower the elevated plasma osmolarity. Since in this case ADH is reduced in the face of elevated osmolarity and the kidney is not reabsorbing water, it is clear that neurogenic diabetes insipidus is present (choice C). Choice A is incorrect because congestive heart failure will trigger water retention via the reduced cardiac output and lower arterial pressure, which stimulates ADH release via arterial baroreceptors. Choice B is incorrect because nephrogenic diabetes insipidus results from renal insensitivity to normal, or elevated circulating ADH levels. Choice D is incorrect because primary polydipsia is associated with reduced plasma osmolarity which will suppress ADH release. Choice

E is incorrect because diabetes mellitus will be associated with elevated plasma and urine glucose levels.

Correct Answer: E

Section: Biochemistry Eukaryotic viral RNAs are not capped and the host 40S ribosome interacts with these viral RNAs at an IRES. Capped eukaryotic mRNAs are recognized by the complex of eIF- 4E and eIF-4G, which in turn allows for interaction of the 40S ribosome with the mRNA (see below figure).

Initiation factor eIF-4E physically binds the cap structure in eukaryotic mRNAs and this function is facilitated by interaction of eIF-4E with eIF-4G. Thus, loss of the interaction of eIF-4E with the protease cleaved eIF- 4G results in loss of translational initiation from capped mRNAs. However, the binding of 40S ribosomes to viral RNAs does not require eIF-4E, only eIF-4G. The protease encoded by the poliovirus cleaves the eIF- 4E binding site on eIF-4G. None of the other translation factors (choices AC, E) are targets for poliovirus protease.

Correct Answer: A

Section: Microbiology/Immunology Adenoviruses are the only ones that produce spikes extending from each one of the vertices of the capsid. These microbes are DNA, nonenveloped, icosahedral viruses. Adenoviruses causepper respiratory infections associated with coryza, sore throat, fever, and conjunctivitis

Correct Answer: C

Section: Microbiology/Immunology The NA inhibitors zanamivir and oseltamivir (tamiflu) were approved in 1999 for the treatment of influenza Aand influenza B. Amantadine (generic name) or Symmetrel (trade name) inhibits an early event in the multiplication cycle of influenza virus, as well as arenaviruses. It blocks the uncoating process. Mutations in the M protein genes result in the development of drug-resistant mutants. The drug is not used extensively in the United States because it seems impractical to control this type of infectious disease that is not ordinarily fatal. To protect individuals at high risk, and in those in whom the infection is potentially dangerous, the choice is between this drug and the influenza vaccine. In most cases, the vaccine is usually preferred. Acyclovir is used to shorten the duration of herpes simplex virus episodes and also to limit the duration of viral shedding (choice B). There are no antiviral drugs for rabies (choice D). Acyclovir has slight antiviral action for EBV (choice A). There is no antiviral agent against rhinovirus (choice E).

Correct Answer: B

Section: Microbiology/Immunology Botulism is worldwide in distribution and is caused by C. botulinum (choice B). C. botulinum is an anerobic, spore-producing grampositive organism. During the growth of the vegetative form of the organism, toxins (types AG) are liberated into the environment, especially when the bacteria undergo autolysis. Illness in humans is most often caused by ingestion of foods contaminated with the toxin. Botulism toxin is absorbed from the gut and binds to receptors of presynaptic membranes of motor neurons of the peripheral nervous system and cranial nerves. This inhibits the release of acetylcholine, producing a flaccid paralysis. Use of antibiotics would be inappropriate and useless since no living bacteria are involved in this presentation of disease. Use of antitoxins would be the method of choice for intervention. B. anthracis (choice A) causes anthrax by bacterial spores entering small abrasions in oral mucosa and germinating into vegetative bacteria. Antibiotics would be the treatment of choice in a clinical situation, although vaccines are available for domestic animals. The other three choices (C, D, and E) all include clostridial organisms that cause diseases where antibiotic treatment is necessary.

Correct Answer: B

Section: Pathology and Path physiology The fine-needle biopsy shown in figure demonstrates a fairly monomorphic population of small, mitotically active neoplastic lymphocytes with associated starry sky histiocytes. The morphology of the tumor and the clinical history are typical for endemic Burkitt lymphoma. The diagnosis of adenocarcinoma (choice A) is incompatible with the photograph because malignant epithelial elements are not seen. Chronic lymphocytic leukemialymphoma (choice C) is distinctly uncommon in children and features mature lymphocytes without a starry sky background. There are no parasitic elements (choice D) evident in the photograph. Salivary gland lymphoepithelioma (choice E) is usually seen in an older age group, contains some remaining epithelial elements, and has larger immunoblastic-type lymphocytes.

Correct Answer: A

Section: Pharmacology Alendronate is a bisphosphonate, the class of drugs that has proved most effective in the treatment and prevention of osteoporosis. This class of drug is also useful in the management of hypercalcemia and certain neoplasms. Calcitonin (choice B) has been shown to slow bone loss in postmenopausal women and to treat postoperative pancreatitis. Fluoride (choice C) is used primarily in the prevention of dental caries. Parathyroid hormone (choice D), marketed as Forteo, is currently used in the clinic to treat osteoporosis. Vitamin D (choice E) has shown benefit in increasing bone deposition in osteoporotic patients. However, choices BE do not have clinical use in the treatment of malignancy or Paget disease.

Correct Answer: E

Section: Pharmacology Pramipexole is a dopamine receptor agonist unrelated to levodopa or ergot derivatives such as bromocriptine and pergolide (which are also useful dopamine agonists). Amantadine (choice A) acts to facilitate the release or slow the reuptake of dopamine, but does not act on the dopamine receptor. Carbidopa (choice B) blocks the peripheral conversion of levodopa to dopamine, it does not act on dopamine receptors. Entacapone (choice C) blocks the peripheral metabolism of levodopa by catechol- Omethyltransferase. Orphenadrine (choice D) is a centrally acting anticholinergic agent that is sometimes used in Parkinson's disease.

Correct Answer: A

Section: Pharmacology Allopurinol and its metabolite alloxanthine inhibit xanthine oxidase, thus preventing conversion of xanthine and hypoxanthine to uric acid. Although xanthine and hypoxanthine then accumulate, these compounds are more soluble than uric acid and less likely to deposit in joints or precipitate in the urine. Most doses of aspirin (choice B) increase retention of uric acid, especially low doses. Colchicine (choice C) is an inhibitor of microtubule function that brings relief in an acute gout attack by inhibiting the motility of granulocytes and preventing the formation of mediators of inflammation by leukocytes. Because of its toxicity at higher doses, it is now used chiefly at low doses to prevent acute attacks. Indomethacin (choice D) is an NSAID that inhibits COX and reduces formation of prostaglandins and eicosanoids involved in gouty arthritis. It has no effect on the formation of uric acid and very little on its excretion. Sulfinpyrazone (choice E) and probenecid are uricosuric agents--they increase the excretion of uric acid by the kidney. Renal uric acid excretion is determined by the balance between the amount filtered plus that actively secreted and the amount undergoing passive and active reabsorption. At very low doses, these agents inhibit active secretion and thus promote retention of uric acid. At higher (clinical) doses, both active secretion and active reabsorption are inhibited, with the result that excretion is enhanced.