USMLE-STEP-1 Online Practice Questions and Answers

Correct Answer: E

Section: Anatomy The retina contains three layers of cells. They are, from top to bottom, the ganglion cells (choice A), the bipolar cells (choice C), and the photoreceptor cells (rods and cones, choice E). The internal plexiform layer (choice B) contains the synapses between the bipolar cells and the ganglion cells. The external plexiform layer (choice D) contains the synapses between the photoreceptors and the bipolar cells. Remember that light enters from the top and traverses all the layers to reach the photoreceptors in the bottom layer.

Correct Answer: A

Section: Physiology Type I diabetes mellitus is a condition in which the pancreas secretes inadequate quantities of insulin, causing a fasting hyperglycemia. Choice Adepicts the filtered glucose in such a hyperglycemic state. Choices B and C depict the reabsorbed and excreted glucose which would also occur in a hyperglycemic state. Choice D depicts the filtered and reabsorbed glucose in a fasting, normoglycemic state. Choice E depicts the zero excreted glucose that would occur in a fasting nondiabetic individual since the tubular maximum for glucose has not been exceeded.

Correct Answer: D

Section: Biochemistry Two familial syndromes directly involve defects in LCAT. Familial LCAT deficiency is characterized by near complete absence of the enzyme activity from the plasma. Fish eye disease is characterized by an absence of LCAT activity toward high-density lipoproteins (HDLs) but presence of activity toward LDLs. Clinical features of familial LCAT deficiency include corneal opacities, anemia, and proteinuria. Due to the lack of LCAT activity, the plasma level of esterified cholesterol is lower than normal and phosphatidylcholine (the principal source of fatty acid for esterification to cholesterol) levels are higher than normal. The profile of all classes of plasma lipoproteins in patients with familial LCAT deficiency is abnormal. Bassen-Kornzweig syndrome (choice A), also identified as abetalipoproteinemia, is due to a defect in apo B expression. Clinical symptoms include retinitis pigmentosa, ataxic neuropathy, and erythrocytes that appear thorny (acanthocytosis). FH (choice B) is characterized by reduced LDL clearance, which leads to severe hypercholesterolemia. Major clinical symptoms are arterial deposition of LDL-cholesterol, which leads to atherosclerosis and coronary artery disease. Deposition of LDL-cholesterol is also seen in tendons and skin resulting in xanthomas. Familial hypertriacylglycerolemia (choice C), also identified as hyperlipoproteinemia type IV, is a form of LPL deficiency. The defect leads to increased levels of circulating VLDLs and is associated with glucose intolerance and hyperinsulinemia. This disorder is frequently associated with Type II diabetes. Wolman disease (choice E) is cholesterol ester storage disease that leads to massive accumulation of cholesteryl esters and triglycerides in most tissues. This disease is almost always fatal within the first year of life and thus would not be present in a 28-year- old.

Correct Answer: C

Section: Biochemistry The lac operon in bacteria is a group of three coordinately regulated genes whose functions are to allow the bacterium to survive when glucose is low or absent as an energy source. The major nonglucose source of energy is from lactose, a disaccharide of glucose and galactose. The control of expression from the lac operon by glucose is exerted through the level of cAMP, whereas, control by lactose is exerted through induced inhibition of the repressor of the operon. When glucose levels are plentiful, the level of cAMP is low. The role of cAMP in the regulation of the lac operon is to bind to the catabolite activator protein (CAP), which is a DNA-binding factor that stimulates the activity of RNA polymerase. As glucose levels drop, cAMP levels rise with a concomitant increase in CAP-mediated activation of polymerase. In the presence of lactose, the lac operon repressor protein is prevented from binding to the operator region of the operon resulting in an increase in the rate of transcription. Therefore, maximal activity would be seen with no glucose in the presence of lactose. With both glucose and lactose present (choice A), the level of cAMP would be low so there would be no activation of polymerase via CAP, but some level of transcription would occur due to the lactosemediated inhibition of the repressor activity. With glucose and no lactose (choice B) the lac operon would be maximally repressed since CAP would be inactive and repressor would be uninhibited. With no glucose or lactose (choice D), the level of cAMP would be high so activation of CAP would be evident leading to transcription, but due to the presence of the repressor the rate of transcription would not be maximal.

Correct Answer: E

Section: Biochemistry Hereditary orotic aciduria results from a defect in the de novo synthesis of pyrimidines. The defect is in the bifunctional enzyme that catalyzes the last two steps in the de novo pathway, conversion of orotic acid to OMP and OMP to UMP. Administration of uridine allows afflicted individuals to produce sufficient levels of cytidine nucleotides via the salvage pathways. Treatment with uridine leads to a return of normal blood hemoglobin levels, and bone marrow will become normoblastic. Treatment with cytidine (choice B) has some limited ability to ameliorate symptoms of the disease, but not to the extent of uridine administration. None of the other nucleosides (choices A, C, and D) can be salvaged into cytidine or uridine nucleotides, and are therefore of no clinical value in the treatment of hereditary orotic aciduria.

Correct Answer: C

Section: Biochemistry Translation in a eukaryotic in vitro translation system would begin at an AUG codon residing near the 5'end of the mRNA. Therefore, translation of the correct RNA product would begin two nucleotides from the 5' end at the first AUG codon. This would result in the translation of a protein of four amino acids. None of the other choices (A, B, D, and E) translate into a four-unit amino acid chain.

Correct Answer: A

Section: Microbiology/Immunology The spleen is important for the production of IgM and IgG antibodies needed to opsonize encapsulated bacteria. Involvement of the spleen in metabolizing (choice D), inactivating of antimicrobial agents (choice E), or processing of capsular polysaccharides (choice C) is not considered important. The main source of stem cells is the bone marrow not the spleen (choice B).

Correct Answer: D

Section: Pathology and Path physiology Laboratory findings of a hypochromic anemia with a decrease in MCV and MCHC indices and a decreased reticulocyte count suggest a hypoproliferative anemia. The various microcytic, hypochromic anemias belong to this category and a partial differential diagnosis for these includes iron-deficiency anemia, anemia of chronic disease (ACD) (choice A), thalassemias, and sideroblastic anemias. ACD (choice A) may be distinguished from irondeficiency anemia by additional iron studies, as were performed in this case. While serum iron and ferritin levels are low and total ironbinding capacity (TIBC) is elevated in irondeficiency anemia, in ACD ferritin is typically normal or elevated, iron is low, and TIBC is low. ACD is associated with a functional, not an absolute, deficiency in iron as a result of sequestration by reticuloendothelial cells. Aplastic anemia (choice B) causes a pancytopenia, which was not the case in this patient as WBC and platelet levels were normal. Hypothyroidism (choice C) and pernicious anemia (choice E) are megaloblastic anemias, which demonstrate increased, not decreased, MCV values (~ >110 fL).

Correct Answer: D

Section: Pathology and Path physiology This infant has cretinism, a condition caused by a neonatal lack of thyroxine. Thyroid agenesis, iodine deficiency, ingestion of goitrogens, and hereditary enzymatic deficiencies may all result in a relative lack of biologically active thyroxine. Affected children may display lethargy, jaundice, hypothermia, muscular hypotonia, and mental retardation. Medicinal replacement of thyroxine is therapeutic. The mental retardation may not be reversible, however, unless treated early. Antibiotics (choice A) have no effect in treating cretinism. Growth hormone (choice B) is an effective treatment for pituitary dwarfism but it has no benefit in cretinism. Treatment with thiamine (choice C) is the appropriate therapy for beriberi, not cretinism. A lack of vitamin D (choice E) causes rickets, not cretinism.

Correct Answer: E

Section: Pharmacology Leuprolide is a GnRH agonist that when given in pulsatile fashion stimulates pituitary LH release, but when given continuously, or in a depot preparation, suppresses LH release and reduces plasma testosterone levels to castration levels. Because it is a peptide, it must be given parenterally. It is used extensively in the treatment of metastatic prostate cancer. Diethylstilbestrol (choice A) is an oral nonsteroidal synthetic estrogen that is now rarely used in the treatment of prostate cancer. Use of this agent produces undesirable feminization and is reserved for cases unresponsive to other agents. Finasteride (choice B) is an orally active inhibitor of the 5-alpha-reductase that converts testosterone to dihydrotestosterone in the prostate. Because conversion of testosterone to dihydrotestosterone is essential for the androgenic effects in the prostate, inhibition of the 5-alpha-reductase decreases the stimulation of the prostate. Unfortunately, the drug has limited efficacy in prostate malignancy. Finasteride is used for the treatment of benign prostatic hyperplasia because it produces a moderate decrease in prostate size and improvement in urinary flow. It is also used for the treatment of androgenic hair loss. Flutamide (choice C) is a nonsteroidal antiandrogen occasionally used in the treatment of prostate cancer that is unresponsive to leuprolide. It is active orally. Ketoconazole (choice D) is an oral antifungal agent that has the property of inhibiting the cytochrome P450 isoforms involved in steroid biosynthesis. It has been used in the treatment of prostate cancer with mixed results.

Correct Answer: D

Section: Pharmacology The transplanted heart lacks functional innervation for at least 2 years after surgery, and possibly longer. Because the nerves are cut in the procedure, nerve endings degenerate (Wallerian degeneration) and transmitter stores are lost. Adrenoreceptors on the denervated myocardial cells are normal or even increased in sensitivity, so responses to direct-acting betaadrenoreceptor agonists such as isoproterenol are retained. Indirectly acting sympathomimetics, on the other hand, are relatively ineffective (choices A, B, and C) because they act through the release or amplification of endogenous

Correct Answer: C

Section: Behavioral Science and Biostatics The anal phase of development occurs around age 2, around the time of toilet training. Developmental difficulty around this time may result in "anal-phase fixation," in which some characteristics of this phase may unduly influence the adult personality. Such characteristics include struggles over the issue of control.

Excessive cleanliness and orderliness, or the opposite, as well as parsimony and hoarding may occur. Dependency (choice A) is characteristic of oral phase, and fierce competitiveness (choice B) may be characteristic of the Oedipal phase. Narcissism (choice D) is a complex phenomenon that develops from very early life and not confined to any particular stage of development.

Correct Answer: B

Section: Behavioral Science and Biostatics Although all the other items (choices A, C, D, and E) may be significant, schizophrenia cannot be diagnosed if the patient has never had an episode of psychotic symptoms lasting for at least 6 months.

Correct Answer: D

Section: Behavioral Science and Biostatics Paroxetine is a selective serotonin reuptake inhibitor (SSRI), an antidepressant, and is the drug of choice for treating this patient's depression. Carbamazepine (choice A), like valproic acid (choice E), is an anticonvulsant and mood stabilizer which may be used to treat bipolar disorder. Methylphenidate (choice B) is a CNS stimulant; olanzapine (choice C) is an atypical antipsychotic.

Correct Answer: D

Section: Physiology Nitric oxide has important neurotransmitterlike functions. Its short half-life, due to spontaneous decay, limits its range and action. Nitric oxide acts on smooth muscle cells in a generally inhibitory manner; it relaxes the GI muscles and sphincters and dilates blood vessels. In blood vessels, nitric oxide is derived from endothelial cells and has been identified as the long-hypothesized endothelium-derived relaxing factor (EDRF). Acetylcholine (choice A) is the classic neurotransmitter of the parasympathetic nervous system. Although activation of the pelvic parasympathetic nerves leads to erection, it is nitric oxide and not acetylcholine that relaxes penile artery smooth muscle cells. GABA (choice C) is an inhibitory neurotransmitter found in the central nervous system. Epinephrine (choice B) and norepinephrine (choice E) play a role during the ejaculation phase of the male sexual act, but do not contribute to penile artery dilation during the erectile phase.