USMLE USMLE-STEP-1 Online Practice Questions and Exam Preparation

USMLE USMLE-STEP-1 Online Questions & Answers

• Question 91:

  Amother has brought her 3-month-old baby to the pediatrician and indicates that the infant is lethargic and has poor suckling and seems uninterested in eating. In addition, the mother notes that the baby's diapers often smell like burnt sugar. This infant likely has a defect in which of the following enzymes?

  A. branched-chain alpha-keto acid dehydrogenase
  B. cystathionine synthase
  C. glycine cleavage complex (GCC)
  D. homogentisate oxidase
  E. phenylalanine hydroxylase
  A. branched-chain alpha-keto acid dehydrogenase

• Question 92:

  The process of RNA editing leads to sequence changes that are not reflective of the genetic information contained within the gene for an edited RNA. RNA editing occurs through the action of specific nucleotide deaminases that change either adenine to inosine or cytidine to uridine. These nucleotide changes can affect the coding or splicing capacity of the RNA. Which of the following proteins is an important example of the result of RNA editing?

  A. apo B48
  B. calcitonin gene-related peptide
  C. ferritin
  D. glutamate dehydrogenase
  E. tissue factor (factor III)
  A. apo B48

• Question 93:

  Desmin is a 53-kDa protein found in skeletal and smooth muscle cells. It plays a role in coordinating muscle cell contraction. Desmin belongs to which type of intermediate filaments?

  A. type I
  B. type III
  C. type IV
  D. type V
  E. type VI
  B. type III

  ---

  Explanation

  Section: Anatomy Desmin belongs to the type III intermediate filaments. Intermediate filament proteins have three domains: a head domain (N-terminal), an alpha-helical rod domain, and a tail domain (C-terminal). The six types of intermediate filament proteins are differentiated by the sequence in the rod domain. Type I (choice A) intermediate filaments include the cytokeratins. Type IV (choice C) intermediate filaments are found in the nervous system and include the neurofilaments and alpha-internexin. Type V (choice D) intermediate filaments are the nuclear lamins. Nestin, expressed in stem cells of the central nervous system, is a type VI (choice E) intermediate filament.

• Question 94:

  The protein encoded by the APC gene is a tumor suppressor the role of which is to regulate the activity of cellular signaling induced by the Wnt growth factor. Therefore, loss of APC activity is associated with unrestrained cellular proliferation. Which of the following signaling molecules is the target for APC interaction?

  A. beta-catenin
  B. MYC
  C. p53
  D. pRB
  E. von Hippel-Lindau protein (pVHL)
  A. beta-catenin

  ---

  Explanation

  Section: Biochemistry The protein of the Wnt signaling cascade, to which APC associates, is beta-catenin. Using antibodies specific for the NH2-terminus of APC, it was possible to coprecipitate additional PCassociated proteins leading to the identification that one of these APC-associated proteins was beta-catenin. The catenins are a family of proteins that interact with the cytoplasmic portion of the cadherins (cell-cell adhesion family of proteins), thus linking the cadherins to the actin cytoskeleton. Catenins are equally important in the signaling cascade initiated by the Wnt family of proteins that are involved in embryonic patterning, development of the nervous system. The Wnt proteins are secreted factors that interact with cell-surface receptors. Wntreceptor interaction induces the activity of the cytoplasmic phosphoprotein dishevelled. Activated dishevelled inhibits the serine/ threonine kinase glycogen synthase kinase-3- beta (GSK-3). When GSK-3-beta is inhibited, beta-catenin becomes hypophosphorylated. The hypophosphorylated form of beta-catenin migrates to the nucleus and interacts with transcription factors, thereby, inducing expression of various genes. The role of APC in this pathway is to bind phosphorylated beta-catenin. The APC-beta-catenin complex stimulates the breakdown of beta-catenin. Therefore, mutations that lead to a loss of APC, or to a loss of the portion of the APC protein that interacts with beta-catenin, would lead to constitutive activation of several genes that could then promote the transformed phenotype. None of the other proteins (choices BE) are known to interact with APC.

• Question 95:

  A Gram-stain smear of spinal fluid from a 2- year-old child reveals short gram-negative rods. It grows on enriched chocolate agar, but does not grow on blood agar, except adjacent to a streak of staphylococci.

  The organism most probably is which of the following?

  B. N. gonorrhoeae
  C. N. meningitides
  D. L. monocytogenes
  E. S. pneumoniae
  H. influenza
  A

  ---

  Explanation

  Section: Microbiology/Immunology

  The organisms of the genus Haemophilus are small, gram-negative, nonmotile, nonspore-forming bacilli with complex growth requirements. H. influenzae requires a heatstable factor found in blood (X factor), which can be replaced by hematin, and nicotinamide adenine dinucleotide (V factor), which can be added to the medium as a supplement, or can be supplied by other microorganisms, such as staphylococci (satellite phenomenon). N. meningitides (choice C), N. gonorrhoeae (choice D), S. pneumoniae

  (choice E), and L. monocytogenes (choice B) are able to synthesize hematin and nicotinamide adenine dinucleotide, and thus they will grow in culture media which do not contain these nutrients. They also do not need to rely on the production of hematin and nicotinamide adenine dinucleotide by S. aureus to grow. In contrast to H. influenzae, S. pneumonia is a gram-positive, lancet- shaped diplococcus, not a gram-negative bacterium.

  L. monocytogenes is a gram-positive rod.

• Question 96:

  Which of the following represents the enzyme deficiency that leads to "essential fructosuria"?

  A. fructose-1-phosphate aldolase (aldolase B)
  B. fructose-1,6-bisphosphate aldolase (aldolase A)
  C. fructokinase
  D. hexokinase
  E. 6-PFK-1
  C. fructokinase

  ---

  Explanation

  Section: Biochemistry Essential fructosuria is an autosomal recessive disorder manifesting benign asymptomology due to a lack of fructokinase. The principal clinical signs of essential fructosuria are hyper-fructosemia and fructosuria. Deficiency in aldolase B (choice A) results in the clinically severe disorder, hereditary fructose intolerance. Symptoms include severe hypoglycemia and vomiting after ingestion of fructose. Prolonged fructose ingestion by infants with this disorder will lead to poor feeding, hepatomegaly, vomiting, jaundice, and eventually hepatic failure and death. Aldolase A deficiency (choice B) is very rare and has only been observed associated with erythrocytes and fibroblasts and leads to hemolytic anemia. Hexokinase deficiency (choice D) is also rare and associated with erythrocyte disfuction leading to chronic hemolytic anemia. Deficiency in PFK-1 (choice E) results in the glycogen storage disease known as Tarui disease. Symptoms of PFK-1 deficiency include exercise-induced cramping and pain, myoglobinuria, and hemolytic anemia.

• Question 97:

  Acetyl-CoA enhances the rate of gluconeogenesis by acting as an allosteric activator of which of the following enzymes?

  A. ACC
  B. PEP-carboxykinase
  C. pyruvate carboxylase
  D. PDH
  E. pyruvate kinase
  C. pyruvate carboxylase

  ---

  Explanation

  Section: Biochemistry The major substrates of gluconeogenesis are pyruvate and lactate. During gluconeogenesis lactate is oxidized to pyruvate. For pyruvate to be converted back to glucose, it must first be carboxylated to oxaloacetate, since a reversal of the PK reaction of glycolysis cannot occur to convert the pyruvate to phosphoenolpyruvate. The carboxylation of pyruvate is catalyzed by the mitochondrial enzyme pyruvate carboxylase. The activity of pyruvate carboxylase is absolutely dependent on the presence of acetyl- CoA, which allosterically activates the enzyme. Of the enzymes listed, only pyruvate carboxylase and PEPCK (choice B) are involved in gluconeogenesis. ACC (choice A) is involved in fatty acid synthesis and is not regulated by acetyl-CoA. PDH (choice D) is the entry point for pyruvate into the TCA cycle. It is also inhibited by acetyl- CoAvia the acetyl-CoA-mediated activation of PDH kinase, an enzyme that phosphorylatesand inactivates PDH. PK (choice E) is a glycolytic enzyme and is inhibited by acetyl- CoA, not activated by it.

• Question 98:

  A 73-year-old man with a history of an MI 2 years ago has been experiencing gradually increasing shortness of breath over the past 4 weeks. Physical examination reveals bilateral crackles on pulmonary auscultation and 2+ pitting edema of the lower legs bilaterally. Which of the following most likely occurred in this patient to produce these changes?

  A. interventricular septal wall thickening
  B. lobar pneumonia
  C. new MI
  D. spontaneous idiopathic pneumothorax
  E. ventricular aneurysm rupture
  C. new MI

  ---

  Explanation

  Section: Pathology and Path physiology Bilateral crackles on lung auscultation and 2+ pitting edema of the bilateral lower extremities are signs of left-sided and right-sided congestive heart failure, respectively. The only logical choice that could lead to both in this patient is an MI, where left ventricular failure subsequently leads to right ventricular failure. Interventricular septal wall thickening (choice A) is a particular characteristic of idiopathic hypertrophic cardiomyopathy, and frequently causes outflow tract obstruction during systole. It is an autosomal- dominant genetic disease leading to serious progressive cardiac abnormalities typically beginning in young adulthood, and is a common cause of sudden, otherwise unexplainable, death in young athletes. While lobar pneumonia (choice B) is common in older patients, and could lead to shortness of breath, it would not be associated with right heart failure and bilateral lower extremity edema. Similarly, a spontaneous idiopathic pneumothorax (choice D) may also result in shortness of breath, but does not cause right heart failure; this phenomenon is most commonly encountered in young individuals, not an older population. Ventricular aneurysms (choice E) occur as a possible complication of extensive transmural anteroseptal MIs in which a large area of thin scar tissue produces external bulging; they are not associated with rupture.

• Question 99:

  A retired policeman who received a kidney transplant developed a generalized infection by an enveloped double-stranded DNA virus. This organism formed owl's eye inclusions in cells found in urinary sediments. Which of the following viruses is the most likely etiological agent?

  A. adenovirus
  B. coronavirus
  C. cytomegalovirus
  D. papillomavirus
  E. variola virus
  C. cytomegalovirus

  ---

  Explanation

  Section: Microbiology/Immunology Cytomegalovirus (CMV) is an enveloped, double-stranded DNA virus, which causes formation of giant cells with intranuclear inclusion bodies that have an oval owl's eye shape. CMV can cause a generalized infection, which is associated with immunosuppression. This can occur with organ transplantation or AIDS and result in central nervous system involvement, pneumonia, retinitis, colitis, and organ rejection. Adenoviruses (choice A) are also double-stranded DNA viruses but do not have an envelope.

  Coronaviruses (choice B) contain an RNA genome and do have an envelope. Papillomaviruses (choice D) contain doublestranded DNA and have no envelope. Variola virus (choice E) is a poxvirus, having doublestranded DNA. The largest virus group by size, poxviruses, does contain an "envelope" but it is constructed of multiple membranes closely attached to the virions, unlike lipid envelopes seen in herpes viruses, for example.

• Question 100:

  A student has a genetic defect and cannot produce the J chain which is important in the structure of some immunoglobul in molecules. Which of the following will most likely be observed in this individual?

  A. a decrease in mature B cells
  B. a decrease in mature T lymphocytes
  C. a decrease in serum IgM
  D. an increase in IgA in the intestine
  E. an increase in serum IgM and decrease in IgE
  C. a decrease in serum IgM

  ---

  Explanation

  Section: Microbiology/Immunology Deficiency in production of J chains for immunoglobulins could result in a decrease in serum IgA and IgM levels. These antibodies are dimers and pentamers, respectively, of the basic immunoglobulin molecule, and require a J chain to join the immunoglobulin chains. There is no evidence which indicates that a deficiency in the production of J chains of IgA or IgM will cause a decrease in mature B cells, or T lymphocytes, an increase in serum IgM and decrease in IgE, or a decrease in IgA (choices A, B, D, and E).