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

USMLE USMLE-STEP-1 Online Questions & Answers

• Question 121:

  A 58-year-old nurse has a herpetic infection that requires administration of acyclovir. Which one of the following is the known mechanism of action of acyclovir?

  A. degradation of viral RNA
  B. inhibition of reactivation of virus in trigeminal root ganglia
  C. inhibition of viral RNA synthesis
  D. killing of actively dividing cells in which virus replicates
  E. termination of DNA replication of herpes virus
  E. termination of DNA replication of herpes virus

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  Explanation

  Section: Microbiology/Immunology Acyclovir is a guanosine analogue which inhibits viral DNA synthesis. It is activated in cells by the virus- encoded thymidine kinase, which phosphorylates this drug. Only viruses that produce thymidine kinase, such as herpes simplex 1 and 2 or the varicella-zoster virus are susceptible to acyclovir. This antiviral agent has not been shown to inhibit reactivation of herpes virus in trigeminal ganglia (choice B), to kill actively dividing cells in which virus replicates (choice D), to degrade (choice A), or to inhibit viral RNA synthesis (choice C).

• Question 122:

  According to psychoanalytic theory, which of the following is a function of the superego?

  A. curiosity
  B. guilt
  C. music appreciation
  D. perception
  E. sexual urge
  B. guilt

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  Explanation

  Section: Behavioral Science and Biostatics The superego is the collection of functions having to do with conscience, social expectations, and guilt, which arises from failing to meet the demands of conscience. Id is the reservoir of basic instinctual drives, where sexual urge belongs E.. Ego is the collection of functions that mediate relations between the demands of the id and the constraints of social reality and conscience, as well as some innate, nonconflictual spheres of existence such as perception and movement. Choices A, C, and D are ego functions.

• Question 123:

  In adults, lack of vitamin D gives rise to the disease osteomalacia characterized by progressive softening and bending of the bones. This is due to a defect in the mineralization of the osteoid. Under normal conditions, the osteoid is found along which of the following locations?

  A. the interface between osteocytes and bones
  B. the interface between osteoprogenitor cells and bone marrow
  C. the interface between the fibroblasts in the periosteum
  D. the interface between the osteoblast and bone
  E. the ruffled border of osteoclasts
  D. the interface between the osteoblast and bone

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  Explanation

  Section: Anatomy Osteoid is the unmineralized organic matrix formed by osteoblasts and found at the interface between these cells and bone. Osteocytes (choice A) are surrounded by bone and no longer manufacture osteoid. Osteoprogenitor cells (choice B) are similar to stem cells and do not manufacture bone material. Fibroblasts (choice C) are cells of connective tissue forming the periosteum and not bone. Osteoclasts (choice E) are bone-resorbing cells.

• Question 124:

  During a marathon attempt a runner collapses and is admitted with severe acute dehydration. Which of the following is most likely to occur in this patient?

  A. decreased baroreceptor firing rate
  B. decreased plasma osmolarity
  C. high renal water excretion
  D. low plasma ADH levels
  E. low water permeability of collecting duct tubular cells
  A. decreased baroreceptor firing rate

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  Explanation

  Section: Physiology Acute dehydration results in decreased plasma volume, cardiac output, and arterial pressure, which leads to a lower firing rate of the baroreceptors. Plasma osmolarity increases (choice B) because more water than salt is lost in sweat. The increase in plasma osmolarity leads to increased ADH secretion (choice D). High plasma ADH levels cause increased water permeability of collecting duct cells (choice E). Therefore, more water is reabsorbed by the kidneys and renal water excretion is low (choice C).

• Question 125:

  Steroid hormones interact with specific receptors within target cells. The steroid-receptor complexes then regulate the rate of which of the following intracellular processes?

  A. posttranscriptional processing of specific mRNAs
  B. posttranslational processing of specific proteins
  C. replication of DNA
  D. transcription of specific genes
  E. translation of specific mRNAs
  D. transcription of specific genes

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  Explanation

  Section: Biochemistry Steroid hormones are lipophilic and hence freely penetrate the plasma membrane of all cells. Within target cells, steroid hormones interact with specific receptors. These receptor proteins are composed of two domains: a hormone-binding domain and a DNA-binding domain. Following hormone-receptor, interaction the complex is activated and enters the nucleus. The DNA-binding domain of the receptor interacts with specific nucleotide sequences termed hormone response elements (HREs). The binding of steroid- receptor complexes to HREs results in an altered rate of transcription of the associated gene(s). The effects of steroid-receptor complexes on specific target genes can be either stimulatory or inhibitory with respect to the rate of transcription. Complexes of steroid with receptor have no direct effect on posttranscriptional processing of RNA (choice A), posttranslational events (choice B), DNA replication (choice C), or translation (choice E).

• Question 126:

  Which of the following vessels participate in the arterial circulation of the spleen?

  A. afferent arteriole
  B. efferent arteriole
  C. interlobar arteries
  D. interlobular arteries
  E. sheathed arteriole
  E. sheathed arteriole

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  Explanation

  Section: Anatomy The spleen receives its blood supply from the splenic artery, which branches off from the celiac trunk from the abdominal aorta. At the hilum of the spleen, the splenic artery branches off into trabecular arteries, which enter the spleen along the trabeculae and branch in the parenchyma as central arteries running in the white pulp. The central arteries branch into penicillar arterioles, which give rise to capillaries and sheathed arterioles. The capillaries feed the marginal sinuses as well as the red pulp sinuses.

  Afferent (choice A) and efferent (choice B) arterioles, and interlobar (choice C) and interlobular (choice D) arteries are vessels found in the kidney.

• Question 127:

  Presentation of the platelet membrane protein complex, GPIIb-GPIIIa, which binds with von Willebrand factor, is necessary for which of the following?

  A. activation of PKC leading to phosphorylation of myosin light chain and platelet morphology changes
  B. cleavage and activation of high molecular weight kininogen
  C. inducing platelet crosslinking
  D. release of thrombin from platelet granules
  E. stimulation of endothelial and smooth muscle cell interaction resulting in vasoconstriction
  C. inducing platelet crosslinking

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  Explanation

  Section: Biochemistry The platelet membrane glycoprotein complex, GPIIb-GPIIIa, is a receptor for fibrinogen. Presentation of GPIIb-GPIIIa on the surface of activated platelets leads to platelet crosslinking. The cross-linking occurs between activated platelets that are adhered to the endothelial cell surface via interaction of the platelet surface complex GPIa-GPIIa with exposed collagen, as well as between platelets in the local circulation. The effect is to ensure that a loose platelet plug forms rapidly at the site of vascular injury stemming the flow of blood until a fibrin clot can be formed. Activation of platelet PKC (choice A) is the consequence of thrombin binding to platelet thrombin receptors. Cleavage of high molecular weight kininogen (choice B) occurs upon the activation of factor XII. Release of thrombin from platelet alpha granules (choice D) occurs in response to platelet activation. Activation of platelets must first occur in order for GPIIb- GPIIa to be presented on the surface and, thus, this glycoprotein complex is not directly involved in platelet activation and granule release. Vasoconstriction, in concert with hemostasis (choice E), occurs primarily as a result of the action of serotonin released from alpha granules of activated platelets.

• Question 128:

  A 5-year-old boy is brought to a pediatrician by his mother for progressive abdominal girth and poor appetite. His mother is concerned over a "growing lump" she has felt in his abdomen, and physical examination confirms the presence of a large palpable mass in the left upper quadrant. Abdominal ultrasound demonstrates an 18-cm solid mass in the upper pole of the left kidney. Arenal biopsy is taken. Which of the following is the pathologist most likely to see on microscopic examination of the biopsy?

  A. a mixture of proliferating smooth muscle cells, mature fat cells, and abnormalblood vessels
  B. numerous fusiform cysts lined by uniform cuboidal cells and no normal parenchyma
  C. primitive mesenchymal tissue with minimal tubular and glomerular differentiation
  D. sheets of large cells with abundant clear cytoplasm and small uniform nuclei
  E. well-formed papillary frond structures lined by cytologically normal, thickened epithelium
  C. primitive mesenchymal tissue with minimal tubular and glomerular differentiation

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  Explanation

  Section: Pathology and Path physiology A large tumor appearing in the pole of a kidney in a child of this age is almost certainly a Wilms' tumor. The microscopic appearance of this neoplasm can be highly variable with some areas resembling renal blastema, as well as epithelial and stromal areas. One usually sees attempts to form glomeruli and tubules. Amixture of proliferating smooth muscle cells, mature fat cells, and abnormal blood vessels (choice A) describes an angiomyolipoma. Numerous fusiform cysts lined by uniform cuboidal cells and no normal parenchyma (choice B) is the microscopic appearance of infantile polycystic disease. Sheets of large cells with abundant clear cytoplasm and small uniform nuclei (choice D) describe a renal oncocytoma. Well- formed papillary frond structures lined by cytologically normal, thickened epithelium (choice E) is a description that is compatible with a renal papillary adenoma.

• Question 129:

  Which of the following statements concerning skeletal muscle is correct?

  A. active tension depends on the extent of overlap between thick and thin filaments
  B. during contraction, thin filaments shorten and thick filaments maintain constant length
  C. significant passive tension is generated at normal rest fiber length
  D. tension generation depends on myosin attachment to the z line
  E. velocity of shortening is independent of fiber load
  A. active tension depends on the extent of overlap between thick and thin filaments

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  Explanation

  Section: Physiology Tension generation depends on attachment of the myosin head group to actin, this is referred to as cross bridge formation. The amount of tension that can be generated in response to a stimulus depends, in part, on the number of cross bridge attachments that can be formed. Since myosin molecules form the thick filament and actin comprises the thin filament as overlap between thick and thin filaments increases more myosin head groups can bind to actin and more cross bridges can form. Because of this, active tension generation depends on the extent of overlap of thick and thin filaments. Because skeletal muscle is quite compliant, very little passive tension is encountered until the muscle is stretched almost to its maximal length (choice C). During contraction neither the thick nor the thin filaments shortens (choice B). Contraction occurs as thick and thin filaments slide past each other. Velocity of shortening decreases with increasing load (choice E). In skeletal muscle, thin filaments attach to the Z line (choice D).

• Question 130:

  A 73-year-old patient has heart failure that has worsened over the past 12 months. Which of the following drugs is likely to slow progression even though it has no direct positive or negative effects on cardiac contractility?

  A. digoxin
  B. dobutamine
  C. losartan
  D. nesiritide
  E. propranolol
  C. losartan

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  Explanation

  Section: Pharmacology Losartan, an AT1 angiotensin receptor antagonist, has been shown to slow the progression of heart failure, in common with the ACE inhibitors, beta blockers, and spironolactone, which also slow progression of this disease. Positive inotropic agents such as digoxin (choice A) and dobutamine (choice B) do not slow progression although they may be useful for reducing symptoms. Nesiritide (choice D) is a peptide that causes vasodilation and sodium diuresis and may have value in acute but not chronic failure. Propranolol (choice E) probably slows progression of heart failure but it has direct negative inotropic effects on the heart.