USMLE-STEP-1 Online Practice Questions and Answers

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).

Correct Answer: D

Section: Biochemistry Both rod and cone cells contain a photoreceptor pigment in their membranes. The photosensitive compound (photoreceptor) of most mammalian eyes is a complex of protein and an aldehyde form of vitamin A. The protein component is a member of the opsin family called scotopsin. The photoreceptor of rod cells is specifically called rhodopsin (choice C) or visual purple, which is a complex between the protein scotopsin and the 11-cis-retinal form of vitamin A. Intracellularly, rhodopsin is coupled to a specific G- protein called transducin (choice E). When the rhodopsin is exposed to light it is bleached releasing the 11- cis-retinal from opsin. Absorption of photons by 11-cis-retinal triggers a series of conformational changes on the way to conversion all-trans-retinal. The release of opsin results in a conformational change in the photoreceptor. This conformational change activates transducin, leading to an increased GTP-binding by the alpha-subunit of transducin. Binding of GTP releases the alpha- subunit from the inhibitory beta- and gamma-subunits. The GTPactivated alpha-subunit in turn activates an associated phosphodiesterase (choice A), an enzyme that hydrolyzes cyclic-GMP (cGMP) to GMP. Cyclic GMP is required to maintain the Na+ channels (choice B) of the rod cell in the open conformation. The drop in cGMP concentration results in complete closure of the Na+ channels.

Correct Answer: A

Section: Microbiology/Immunology CJD (choice A) is caused by a prion, a protein agent with no detectable nucleic acid for a genome. It is considered to be one of a group of degenerative central nervous diseases called transmissible spongiform encephalopathies. Its incubation period most often lasts for years. CJD in humans begins gradually, with progressive dementia, ataxia, twitching of muscles (myoclous), and leading to death within a year. While cases are rare, there may be some type of familial association. Kuru (choice B) is similar to CJD but was confined to New Guinea. Once consumption of dead relatives was outlawed, the disease disappeared. Herpes simplex meningitis (choice C) results in a fairly rapid onset of symptoms caused by destructionof brain tissue, followed by a mortality rate of 7580%. Acyclovir treatment is usually beneficial. PML (choice D) is caused by JC Virus (JCV), a polyomavirus. It is considered a slow virus disease. Several polyomaviruses are important DNA tumor viruses. PML is seen in about 5% of patients with AIDS. Clinically, demyelization of the CNS results in oligodendrocyte infection with polyomaviruses. SSPE (choice E) is another slow virus disease, caused by a defective variant of the measles virus. This slowly progressive demyelization of the CNS results in death.

Correct Answer: C

Section: Microbiology/Immunology Tetanus toxin produced by C. tetani is a protease that often first affects the masseter muscles. Patients so affected cannot open their mouths and have what is called trismus. Trismus provides an explanation for the term lockjaw used to describe tetanus. Cholera toxin (choice E--V. cholerae) causes fluid and electrolyte loss that leads to severe diarrhea. The clostridial alpha toxin (choice B--C. perfringens) kills cells and produces necrosis. Diphtheria toxin (choice D--C. diphtheriae) is an inhibitor of protein synthesis affecting heart, kidney, and other cells. Protein synthesis is inhibited because diphtheria exotoxin catalyzes the polyADP-ribosylation of eukaryotic elongation factor 2 (eEF2). Botulinum toxin (choice A) cleaves the proteins involved in the release of acetylcholine. This leads to aralysis of ocular, pharyngeal, and respiratory muscles.

Correct Answer: A

Section: Pathology and Path physiology Amyloid is an acellular material that is eosinophilic. After Congo-red staining, there is apple-green dichroism when examined under polarized light microscopy. Calcium salts (choice B) tend to be deeply basophilic, not eosinophilic, with routine stains. Cholesterol deposits (choice C) tend to dissolve out of tissues with routine processing agents and only empty outlines of where the crystals once were are present remain. Myocyte fibrinoid necrosis (choice D) would be moderately cellular and eosinophilic. Congo-red dichroism is not evident. A postinfarctive cicatrix (choice E) also displays a relatively acellular eosinophilic morphology. Congo-red staining would not be dichroic, however.

Correct Answer: E

Section: Pathology and Path physiology The physical description of this boy is that of Kleinfelter syndrome. This condition is not usually diagnosed until after puberty when the secondary sexual characteristics do not develop normally. Most of these individuals are 47, XXY but some may have additional X or Y chromosomes. 45, XO (choice A) is characteristic of a female with Turner syndrome which is usually diagnosed early in life. 45, YO (choice B) is not compatible with life and is not seen. 46, XX (choice C) is the chromosome number of a normal female and 46, XY (choice D) is that of a normal male. Males with 47, XYY (choice F) are above average in height and have an increased incidence of developing severe acne. IQ is normal but there may be mild learning disabilities.

Correct Answer: B

Section: Pathology and Path physiology Hemophilia A is a genetic disorder characterized by very low levels of factor VIII, elevated partial thromboplastin time, normal prothrombin time, normal platelet aggregation with ristocetin, and spontaneous hemorrhages into joints, soft tissues, and mucosal surfaces. Christmas disease (choice A) is an alternative term for hemophilia B. Hemophilia B (choice C) is a hereditary coagulopathy due to a very low level of factor IX. Rosenthal syndrome (choice D) defines a deficiency of factor XI. von Willebrand disease (choice E) is characterized by a mild hereditary bleeding diathesis and abnormal platelet aggregation with ristocetin.

Correct Answer: C

Section: Pathology and Path physiology Individuals with sickle trait are healthy and not anemic. Hemoglobin electrophoresis demonstrates a minor proportion of hemoglobin S and a major proportion of hemoglobin A. Fetal hemoglobin and A2 hemoglobin are usually normal. Sickle trait confers the benefit of protecting erythrocytes from some forms of malarial infection. About 9% of blacks in the United States have sickle trait. In sickle cell disease (choice A), almost all hemoglobin is hemoglobin S. No hemoglobin A is detected and patients have a clinical history of severe anemia. Sickle thalassemia minor (choice B) presents as a chronic microcytic anemia with a major hemoglobin S component and elevated hemoglobin A2. Thalassemia major (choice D) and thalassemia minor (choice E) do not demonstrate hemoglobin S on electrophoresis.

Correct Answer: B

Section: Pharmacology

Cyclosporine and tacrolimus (not listed) are associated with nephrotoxicity as a major adverse effect.

Antilymphocyte globulin (choice A) is associated with foreign protein reactions, including anaphylaxis.

Mycophenolate mofetil (choice C) causes GI disturbances and myelosuppression. Prednisone (choice D)

causes a variety of chronic toxicities conveniently summarized as "Cushingoid effects" but has little acute

toxicity. Sirolimus (choice E) is associated with bone marrow suppression.

Correct Answer: C

Section: Pharmacology The hydroxymethylglutaryl-CoA (HMGCoA) reductase inhibitor lovastatin and the bile acid-binding resin cholestyramine (choice C) lower the concentration of cholesterol in the blood by different mechanisms. Because they act by different mechanisms, combination therapy is at least additive. The combination of chlortetracycline and amoxicillin (choice A) results in antagonism of amoxicillin's antibacterial action. The beta-lactam cell wall synthesis inhibitors such as the penicillins and cephalosporins are bactericidal, but are effective primarily when the bacteria proliferate rapidly. Tetracyclines are bacteriostatic agents that slow or inhibit the growth of bacterial cells by inhibiting protein synthesis. The combination of clomiphene and chorionic gonadotropin (choice B) does not produce any beneficial additive action. Both agents are used to treat female infertility. Clomiphene is an estrogen-receptor partial agonist that functions at the level of the hypothalamus to stimulate release of gonadotropinreleasing hormone (GnRH). The increased release of GnRH results in increased release of the gonadotropins LH and FSH from the anterior pituitary. This results in stimulation of ovulation. Administration of chorionic gonadotropin also stimulates ovulation. Because both preparations function through increases in gonadotropin levels and both are present at their maximum effective concentrations, a beneficial additive effect is unlikely. The combination of pentazocine and morphine (choice D) does not produce a beneficial interaction. Pentazocine exerts its pain-relieving activity by a weak (partial) agonist action at mu-opioid receptors. Morphine is a full agonist at the same receptors. When the two analgesic agents are combined, pentazocine acts as an antagonist for morphine at mu- receptors. The result is precipitation of withdrawal in addicted patients, and dysphoria and loss of morphine analgesia in nonaddicted patients. Succinylcholine and atracurium (choice E) are both skeletal musclerelaxing agents that block muscle contraction by actions at the neuromuscular junction. Succinylcholine is a depolarizing blocker that acts as an agonist at the nicotinic receptor. Atracurium is a competitive antagonist at the same receptor. Because they both act at the same receptor and are present at their effective concentrations, atracurium interferes with the action of succinylcholine.