USMLE-STEP-1 Online Practice Questions and Answers

Correct Answer: A

Section: Anatomy The intercalated disks are specialized junctional complexes found only in cardiac muscle and they appear as dark lines between the muscle fibers. The motor end-plate (choice B) is a specialized group of synapses between the axon terminals of a motor neuron and the sarcolemma of a skeletal muscle fiber. It is not seen in cardiac muscle. The sarcoplasmic reticulum (choice C) is a specialized modification of the smooth endoplasmic reticulum for sequestering calcium ions. The transverse tubule or T tubule (choice E) is an invagination of the sarcolemma, which penetrates the muscle fiber and overlies the surface of the myofibrils. The sarcoplasmic reticulum and T tubule can only be seen in electron micrographs. There is no tendinous junction (choice D) in cardiac muscle.

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: B

Section: Physiology Bronchiole volume contributes to dead space volume, so increasing bronchoconstriction would decrease bronchiolar volume and thus decrease dead space volume. Bronchoconstriction is a major determinant of airway resistance to air flow. Leukotrienes, by increasing airway constriction, would increase resistance to air flow (choice A). Lung elastic tissue and alveolar surface tension determine lung compliance (choice D), which should not be affected by airway leukotrienes exposure. FRC (choice C), the point of mechanical balance between chest wall and lung, is dependent on lung compliance and, similarly, would not be affected by leukotrienes exposure. Any bronchoconstriction would slightly decrease total lung capacity (choice E).

Correct Answer: C

Section: Physiology Chlorpromazine is an antipsychotic drug with anticholinergic action (i.e., it inhibits the effects of parasympathetic stimulation). Parasympathetic stimulation causes copious secretion by nasal, lacrimal, and parotid glands. Consequently, parasympathetic blockade is leading to a dry mouth. Parasympathetic fibers slow the heart rate and anticholinergic drugs cause tachycardia rather than bradycardia (choice A). Functions of the parasympathetic nervous system include decreasing GI sphincter tone, increasing GI motility, and increasing emptying of the rectum and urinary bladder. Parasympathetic blockade would thus cause increased GI sphincter tone (choice B), urinary retention (choice D), and decreased GI motility (choice E) leading to constipation.

Correct Answer: B

Section: Biochemistry There are three reactions of glycolysis that are thermodynamically irreversible. These are the hexokinase (glucokinase), PFK-1, and PKcatalyzed reactions. Reactions that are essentially irreversible in most metabolic pathways are subject to complex regulatory controls and represent rate-limiting steps in the pathway. The primary site of regulation of glycolysis occurs at the level of the PFK-1-catalyzed step. Hence, this reaction is the rate-limiting step in glycolysis. PFK-1 is subject to allosteric control by numerous compounds. Citrate and ATP inhibit the activity of PFK-1, while AMP and fructose 2,6bisphosphate (F2,6- BP) activate the enzyme. The principal control of PFK-1 activity is exerted by alterations in the level of F2,6- BP. This compound is synthesized from fructose-6-phosphate by the bifunctional enzyme, PFK-2/fructose- 2,6-bisphosphatase (PFK-2/F2,6-BPase). PFK-2 (choice C) contains two catalytic domains, one a kinase and the other a phosphatase, the activities of which are affected by the state of phosphorylation. The phosphatase domain is active when the enzyme is phosphorylated and converts F2,6-BP back to F6-P, thereby reducing the levels of this powerful activator of PFK-1. Thus, although the activity of PFK-2 will determine the rate of activity of PFK-1, it is itself not the rate-limiting enzyme in glycolysis. Glyceraldehyde- 3-phosphate dehydrogenase (choice A) and PGK (choice D) are not regulated enzymes of glycolysis. PK (choice E) is regulated during glycolysis, but does not constitute a rate-limiting step.

Correct Answer: E

Section: Biochemistry The principal mineralocorticod, produced by the zona glomerulosa cells of the adrenals, is aldosterone. Synthesis of aldosterone is primarily controlled by the rennin-angiotensin system and is thus involved in control of blood pressure. Aldosterone causes sodium reabsorption by the kidneys, which in turn regulate water balance, which leads to increases in blood pressure by increasing fluid volume. Aldosterone action does not directly lead to decreased carbohydrate metabolism (choice A), is not a sex characteristic determining steroid hormone (choice B), does not result in increased synthesis of androgens (choice C), and does not regulate its own secretion (choice D).

Correct Answer: C

Section: Biochemistry Deficiencies in the purine nucleotide salvage enzyme, HGPRT, cause three overlapping clinical syndromes. The most severe deficiency (patients having less than 1.5% residual enzyme activity) results in debilitating neurologic disability, overproduciton of uric acid, and behavioral abnormalities that include impulsive and self-injurious activities such as biting finger tips and lips. This severe form of HGPRT deficiency is referred to as Lesch-Nyhan disease. The overproduction of uric acid leads to symptoms of gout and the appearance of "orange sand" in the urine. Deficiencies in ADA (choice A), another purine nucleotide salvage enzyme, are the cause of severe-combined immunodeficiency syndrome, SCID. SCID is characterized by a lack of both cell-mediated and humoral immunity. ADSL (choice B) is an enzyme in the pathway of de novo purine nucleotide synthesis. It is also an important component of the purine nucleotide cycle (see below figure). Deficiencies in ADSLcause psychomotor retardation, epileptic seizures, growth retardation, and muscle wasting. Deficiencies in PNP (choice D) result in a form of immunodeficiency characterized by defective cell-mediated responses. Afflicted individuals may also have normal, hyperactive, or reduced humoral immunity. Orotic aciduria is caused by deficiencies in the de novo pathway of pyrimidine nucleotide synthesis. Adeficiency in either of the last two enzymes in the pathway, orotate phosphoribosyltransferase or OMP decarboxylase, leads to orotic aciduria.

Correct Answer: D

Section: Biochemistry When the carbons of fatty acids are oxidized for energy production, the by-product of that process is the two-carbon compound, acetyl-CoA. Acetyl-CoA can then enter the TCA cycle for complete oxidation. Although, several compounds of the TCA cycle can be directed into the gluconeogenic pathway of glucose synthesis, the carbons of acetyl-CoA cannot provide a net source of carbon in that latter pathway. This is due to the fact that, following entry of the two carbons of acetyl-CoA into the TCA cycle, two carbons are lost as CO2 during the subsequent reactions of the cycle. The subcellular compartmentalization of fat oxidation and gluconeogenesis (choice A) has no bearing on net carbon deposition into glucose. Anabolic and catabolic reactions (choice B) are always occuring concurrently in cells but at different rates dependent on cellular status. The site of fat storage (choice C) has no bearing on net incorporation of carbon into glucose. Acetyl-CoA does not inhibit conversion of pyruvate to oxaloacetate (choice E) but acts as an allosteric activator of pyruvate carboxylase, a gluconeogenic enzyme.

Correct Answer: E

Section: Pathology and Path physiology Wilson disease is an autosomal recessive disorder of copper metabolism due to defective biliary excretion of the copper-protein complex ceruloplasmin. Cells of the liver and brain, notably the basal ganglia, are particularly vulnerable to the toxic effects of excessive copper accumulation. Treatment with copper chelating agents, such as penicillamine or triethylene tetramine, has a dramatically beneficial effect. Congenital hepatic fibrosis (choice A) is a rare disorder of unknown etiology. It is most prevalent in India. Peliosis hepatis (choice B) is a rare hepatic circulatory disorder caused by dilation of sinusoids, resulting in small, blood-filled spaces within the liver; the condition is typically asymptomatic. Steroid hormone usage is associated with its development in some instances. Primary sclerosing cholangitis (choice C) is due to chronic inflammation and fibrosis of intra- and extrahepatic bile ductules. The etiology is obscure. Most of those affected also have ulcerative colitis. Reye syndrome (choice D) refers to acute hepatic failure in children following ingestion of aspirin for certain upper respiratory illnesses, particular influenza, or chickenpox.

Correct Answer: A

Section: Pharmacology Diplopia, abnormal gait, and other signs of cerebellar dysfunction are important symptoms of phenytoin toxicity. Other manifestations of toxicity include gingival hyperplasia, nystagmus, and vertigo. Hyperprolactinemia (choice B) is an adverse effect of antipsychotic dopamine antagonists such as the phenothiazines; dopamine inhibits prolactin secretion by the anterior pituitary. Polydipsia and polyuria (choice C) are symptoms of diabetes insipidus. These symptoms may be produced by lithium toxicity during treatment of bipolar depression, and are not associated with phenytoin toxicity. Postural hypotension (choice D) is not an adverse effect of phenytoin but often occurs with levodopa treatment of Parkinson's disease. Rigidity and tremor (choice E) are symptoms of parkinsonism. These symptoms may be produced by dopamine antagonists such as antipsychotic agents, but are not associated with phenytoin toxicity.