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

USMLE USMLE-STEP-1 Online Questions & Answers

• Question 291:

  Which of the following quantitative comparisons of haloperidol (H) and olanzapine (O) is most accurate?

  

  A. Option A
  B. Option B
  C. Option C
  D. Option D
  E. Option E
  B. Option B

  ---

  Explanation

  Section: Pharmacology Haloperidol is an older oral and parenteral antipsychotic drug with high extrapyramidal but low autonomic toxicity. Olanzapine is a new oral antipsychotic agent with low extrapyramidal and autonomic activity.

• Question 292:

  Figure shows the quantal population doseresponse curves for the therapeutic and toxic effects of drugs X and Y. Both drugs are agonists at the same receptor to produce the therapeutic response, and the maximum responses obtained with each agent are the same. The toxicity curve in the figure shows the superimposed toxic response curves for drugs X and Y; they are identical in terms of the concentration dependence. Which of the following statements is most correct?

  

  A. At 1 × 10-5 M both drugs cause adverse effects in 90100% of patients.
  B. Drug X has a larger therapeutic index than drug Y.
  C. Drug X is more efficacious than drug Y.
  D. Drug Y is more potent than drug X.
  E. Drug Y is safer than drug X.
  B. Drug X has a larger therapeutic index than drug Y.

• Question 293:

  Clinical evidence indicates aspirin is effective in the control of numerous chronic conditions such as atherosclerosis. The principal cardiovascular benefit from aspirin is due to its ability to reduce the incidence and severity of thrombotic episodes. The anticoagulant effect of aspirin occurs through its ability to inhibit which of the following activities?

  A. cyclooxygenase
  B. fibrin cross-linking by factor XIIIa
  C. phospholipase
  D. thrombin binding to activated platelets
  E. von Willebrand factor
  A. cyclooxygenase

• Question 294:

  A young resident doing a fellowship in neuropathology is asked to review histological slides from the cerebral cortex of a 79-year-old nursing home resident, who died of multiinfarct dementia. The resident is asked to estimate the density of neurons in the infracted brain area. To prepare himself for the task, he first reviews slides from the normal areas of the cerebral cortex. Referring to following figure,which of the following structures does he correctly identify as neurons?

  

  A. 1
  B. 2
  C. 3
  D. 4
  E. 5
  D. 4

  ---

  Explanation

  Section: Anatomy Routine neuropathological identification of cells in the central nervous system is based on nuclear morphology using a stain such as the hematoxylin-eosin (H&E) stain shown in Fig. 1-10. The largest nuclei belong to neurons. Nuclei of glial cells are smaller and decrease in size from astroglia (choice A), to oligodendroglia (choice B), to microglia (choice C). Endothelial cells of the penetrating vasculature of the brain represent choice E.

• Question 295:

  The structure indicated by arrow 1 in Fig. 1-2 is which of the following vessels?

  A. brachiocephalic artery
  B. left brachiocephalic vein
  C. left common carotid artery
  D. right brachiocephalic vein
  E. superior vena cava
  D. right brachiocephalic vein

  ---

  Explanation

  Section: Anatomy Remember that in viewing axial or transverse CT scans through the body, the right side of the patient is to your left and the left side to your right. In other words, the feet of the patient are toward you and the head away from you. The back of the patient is at the bottom of the image and the front of the patient toward the top. Directional terms are always in reference to the patient. The insert at the bottom right indicates the level of the section. Arrow 1 indicates the right brachiocephalic vein. The left brachiocephalic vein (choice B) is seen as the elongated structure immediately posterior to the manubrium of the sternum and to the left of the right brachiocephalic vein. Immediately posterior to the left brachiocephalic vein is the brachiocephalic artery (choice A, arrow 2). To the left of the latter are the left common carotid artery (choice C) and the left subclavian artery (arrow 3). The superior vena cava (choice E) is not seen at this level because the right and left brachiocephalic veins are still separate.

• Question 296:

  Exhibit:

  

  The action potentials shown in below figure represent those of which kind of cells?

  A. cardiac nodal cells
  B. myelinated motor axons
  C. sensory neurons
  D. skeletal muscle cells
  E. ventricular Purkinje cells
  A. cardiac nodal cells

  ---

  Explanation

  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.

• Question 297:

  Numerous inherited disorders are the result of the expansion of trinucleotide (triplet) repeats either within the coding regions of genes or the untranslated regions of the resultant RNAs. Which of the following diseases has been shown to be caused by triplet expansion?

  A. cystic fibrosis (CF)
  B. Duchenne muscular dystrophy (DMD)
  C. FH
  D. Huntington disease (HD)
  E. Menkes disease
  D. Huntington disease (HD)

  ---

  Explanation

  Section: Biochemistry HD is an autosomal-dominant disorder leading to progressive memory loss, personality changes, and peculiar motor problems such as involuntary movements of the arms and legs. The disease results from the expansion of a CAG triplet in the amino terminus of the HD protein, referred to as huntingtin. The triplet repeat number ranges from 10 to 30 on the normal chromosome and from 36 to 121 on the HD chromosome. There is a general correlation between the length of the repeat and the age of onset of symptoms. CF (choice A) is primarily due to a common mutation (in 70% of cases) that deletes three nucleotides in exon 10 of the CF gene, which codes for the CF transmembrane conductance receptor (CFTR). Over 600 other mutations have been identified in the CF gene. DMD (choice B) results from deletions in one or more of the exons of the DMD gene, which encodes the protein referred to as dystrophin. FH (choice C) results from defects in the gene encoding the LDL receptor. These defects encompass insertions and deletions that can be found throughout the length of the LDLreceptor gene. Menkes disease (choice E) is due to defects in copper absortion leading to defective function of numerous enzymes that need copper as a cofactor.

• Question 298:

  Which of the following correctly defines the term: p ?

  

  A. equilibrium constant for the dissociation of HA to and
  B. ion constant of water
  C. negative log of the concentration of
  D. pH at which a molecule is neutrally charged
  E. pH at which an equivalent distribution of acid and conjugate base exist in solution
  E. pH at which an equivalent distribution of acid and conjugate base exist in solution

• Question 299:

  Persistent fever and neutropenia with persistently negative blood cultures is often caused by which of the following?

  A. fungi
  B. gram-negative organisms
  C. gram-positive organisms
  D. viral infections
  A. fungi

  ---

  Explanation

  Section: Microbiology/Immunology Most fungi reside in nature and are essential in breaking down and recycling organic matter. About 90% of human infections by fungi can be traced to a few dozen species. Most pathogenic fungi are exogenous, their natural habitats being water, soil, and organic debris. The mycoses with the highest incidence-- candidiases and dermatophytosis--are caused by fungi that are part of the usual or normal microbial flora of humans. Most fungal growth is kept at low levels by bacteria that are more successful at using available nutrients, but fungi are efficient opportunists. Often, hosts who become infected with fungi have some serious metabolic or immune disability. Pathogenic fungi do not produce patent toxins, and the mechanisms of fungal pathogenicity are complex and multifocal, often making these infections hard to treat. The relatively low numbers of fungi in the infection and medium requirements make fungi (choice A) more difficult to isolate, identify, and relate to a medical condition. Gram-positive (choice C) and gramnegative (choice B) bacteria will usually be isolated from clinical specimens. Viral isolation (choice D) is routinely performed successfully today. The symptoms of persistent fever and neutropenia (small number of PMNs) reflect a probable infectious disease process. Failure to associate bacteria and/or viruses with an infectious disease process and clinical history might justify consideration of a fungal etiology.

• Question 300:

  If the RNA synthesized were translated in a eukaryotic in vitro translation system, what would be the composition of the resultant peptide?

  

  A. C-T-I-P-Y
  B. H-S-I-A-C
  C. M-L-W-N
  D. T-C-Y-G-M
  E. V-R-Y-R-T
  C. M-L-W-N

  ---

  Explanation

  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.