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

USMLE USMLE-STEP-1 Online Questions & Answers

• Question 271:

  In analyzing a sample of double-stranded DNA, it has been determined that the molar ratio of adenosine is 20%. Given this information, what is the content of cytidine?

  A. 10
  B. 20
  C. 30
  D. 40
  E. 60
  C. 30

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  Explanation

  Section: Biochemistry The molar ratio of adenosine in any molecule of double-stranded DNA will be equivalent to that of thymidine, since these two nucleotides hydrogen bond to form base pairs. Therefore, the total amount of the DNA accounted for in A-T base pairs would be 40%. Since guanosine and cytidine hydrogen bond to form base pairs, they too contribute an equivalent molar ratio in double-stranded DNA. In double-stranded DNA with 40% A-T composition, the molar ratio of cytidine would be half of the remaining 60% of the DNA, or 30%. No other molar ratio (choices A, B, D, and E) could account for the amount of cytidine in a double- stranded DNA with 20% adenosine.

• Question 272:

  A 10-year-old boy has a bacterial pneumonia. The etiologic agent had M type-1 protein and grew on blood agar. The patient's immune system appeared incapable of controlling the infection. Adecrease in which of the following is most directly related to the immunological failure?

  A. antibody activity
  B. cytotoxic T-cell function
  C. IL-4 and IL-10 levels
  D. mast cell degranulation
  E. platelet aggregation
  A. antibody activity

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  Explanation

  Section: Microbiology/Immunology A deficiency in immunoglobulin production or activity is directly related to susceptibility to common extracellular microbes such as streptococci. Cytokines such as the interleukins, IL-4 and IL-10 (choice C), the function of T cells (choice B) or mast cells (choice D), and nonantibody factors (choice E) are not directly involved in increased susceptibility to bacterial infections.

• Question 273:

  A 5-month-old baby presents to the local community clinic with noted oral candidiasis, chronic diarrhea, extensive diaper rash, and an overall failure to thrive. Previously this infant has been seen for multiple severe, recurrent infections from viruses, fungi, bacteria, and protozoa. Chest x-ray reveals an absent thymic shadow. Which of the following immunodeficiencies best explains this baby's recurrent infections?

  A. Bruton agammaglobulinemia
  B. deficiencies in the C5C8 components of complement
  C. deficiency in neutrophil NADPH oxidase
  D. DiGeorge syndrome
  E. severe combined immunodeficiency (SCID)
  E. severe combined immunodeficiency (SCID)

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  Explanation

  Section: Microbiology/Immunology The most serious immunodeficiency is severe combined immunodeficiency disease (SCID). This occurs as a result of a defect in early stem differentiation resulting in an acute deficiency in both T and B cells. Severe and recurrent infections occur early in life from bacteria, viruses, fungi, and protozoa. DiGeorge syndrome (choice D) is due to poor development of thymus that leads to a pronounced deficiency of T cells. In Bruton agammaglobulinemia (choice A) there is virtual absence of B cells and extremely low levels of immunoglobulins. Deficiencies in complement components C5 through C8 (choice B) are associated with enhanced susceptibility to N. gonorrhoeae and N. meningitides. In chronic granulomatous disease (CGD) there is increased susceptibility to infection by S. aureus, or enteric rods. CGD is due to a deficiency in neutrophilic NADPH oxidase, which is associated with the microbiocidal activity of neutrophils (choice C).

• Question 274:

  A 74-year-old woman with left heart failure develops pulmonary edema. This accumulation of fluid is best explained by which of the following mechanisms?

  A. decreased hydrostatic pressure
  B. decreased lymphatic drainage
  C. decreased oncotic pressure
  D. increased hydrostatic pressure
  E. increased lymphatic drainage
  F. increased oncotic pressure
  D. increased hydrostatic pressure

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  Explanation

  Section: Pathology and Path physiology In left-sided heart failure, poor chamber contractility results in impaired systolic outflow, creating an increase in end diastolic volume and a corresponding increase in ventricular hydrostatic pressure. Impaired ventricular emptying causes stasis in the pulmonary circulation, resulting in a similar increase in volume and pressure in the microvasculature. Capillary congestion leads to widening of endothelial intercellular junctions, leakage of fluid into the interstitium/ alveoli, and the formation of pulmonary edema.

  Decreased hydrostatic (choice A) and oncotic (choice C) pressures and increased lymphatic drainage (choice E) theoretically would reduce total blood volume, thus acting to retard, not induce, pulmonary edema. Decreased lymphatic drainage (choice B) and increased oncotic pressure (choice F) could potentially lead to pulmonary edema; however, they would be secondary factors in cases of left heart failure, as increased hydrostatic pressure is directly responsible for the accumulation of fluid in the lungs.

• Question 275:

  The anticancer drug, Taxol, has been an effective chemotherapeutic agent in the fight against ovarian cancer. Which of the following represents the mechanism of action of Taxol?

  A. binds to the activated form of the RAS protein, which in turn interferes with the signaling cascades involving this protein
  B. binds to microtubules, which stabilizes them preventing their shortening and interfering with cell division
  C. interacts with the tumor supressor protein (pRB) encoded by the retinoblastoma susceptibility gene inducing its suppressive activity
  D. interacts with topoisomerase II preventing its role in DNA synthesis, which effectively terminates replication
  E. interferes with steroid hormone receptor interaction with DNA, thus preventing the growth induction by this class of hormone
  B. binds to microtubules, which stabilizes them preventing their shortening and interfering with cell division

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  Explanation

  Section: Biochemistry The compound paclitaxel (the generic name for Taxol) interferes with the normal function of microtubule growth leading to increased stability of the structure. This effect of Taxol destroys the cell ability to use its cytoskeleton in a flexible manner. Specifically, Taxol binds to the tubulin protein of microtubules and locks them in place. The resulting microtubule/Taxol complex does not have the ability to disassemble. This adversely affects cell function because the shortening and lengthening of microtubules (termed dynamic instability) is necessary for their function as a transportation highway for the cell. Chromosomes, for example, rely on this property of microtubules during mitosis. Therefore, the inability of cells to organize and undertake the necessary movements of mitosis prevents cell division. Since cancer cells are rapidly proliferating cells, the action of Taxol inhibits their growth. None of the other choices (A, CE) reflect the actions of Taxol on cells.

• Question 276:

  Amale infant, delivered at 38 weeks' gestation, presents with severe bowing of long bones, blue sclera, and craniotabes at birth. Radiographs show severe generalized osteoporosis, broad and crumpled long bones, beading ribs, and a poorly mineralized skull. Histologic examination of the long bones revealed the trabecula of the calcified cartilage with an abnormally thin layer of osteoid, and the bony trabeculae are thin and basophilic. The symptoms observed in the infant are characteristic of which disease?

  A. Ehlers-Danlos syndrome
  B. Marfan syndrome
  C. occipital horn syndrome
  D. osteogenesis imperfect
  E. scurvy
  D. osteogenesis imperfect

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  Explanation

  Section: Biochemistry Osteogenesis imperfecta consists of a group of at least four types (mild, extensive, severe, and variable). The disorder is characterized by brittle bones and abnormally thin sclerae, which appear blue owing to the lack of connective tissue. The symptoms arise due to defects in two alpha-collagen genes, the COL1A1 and COL1A2 genes. There have been over 100 mutations identified in these two genes. The mutations lead to decreased expression of collagen or abnormal pro1 proteins. The abnormal proteins associate with normal collagen subunits, which prevents the triple helical structure of normal collagen to form. Theresult is degradation of all the collagen proteins, both normal and abnormal. Ehlers-Danlos syndrome (choice A) comprises at least 10 defined types of a related disorder. Characteristic clinical features are easy bruising, markedly soft hyperextensible skin, extreme joint hypermobility, and the formation of thin, atrophic, "cigarette-paper" scarring following injury. Marfan syndrome (choice B) results in cardiovascular, musculoskeletal, and ophthalmic abnormalities. Hallmark clinical manifestations are aortic dilation, mitral valve prolapse, dissecting aneurysms, arachnodactyly, and ectopia lentis. Occipital horn syndrome (choice C), a disorder that manifests with symptoms similar to other collagen metabolism disorders, results from defects in copper metabolism. Clinical features include loose skin and joints, hernias, and abnormally shaped bones. Scurvy (choice E), which is caused by a deficiency in vitamin C, is characterized by decreased wound healing and hemorrhaging, anemia, osteoporosis, soft swollen gums, and easily bruised skin.

• Question 277:

  Which of the following statements is most correct regarding the use of drugs in hypertension?

  A. ACE inhibitors are contraindicated if the patient is pregnant
  B. beta-blockers are contraindicated if hypertension is associated with heart failure
  C. calcium channel blockers are contraindicated if the patient also has obstructive pulmonary disease
  D. diuretics are contraindicated in hypertension associated with diabetes
  E. vasodilators are contraindicated in a patient taking a diuretic
  A. ACE inhibitors are contraindicated if the patient is pregnant

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  Explanation

  Section: Pharmacology ACE inhibitors are absolutely contraindicated in pregnancy because they cause renal malformations or agenesis in the fetus. Beta blockers (choice B) are very useful in chronic heart failure with or without hypertension. Calcium channel blockers (choice C) have no detrimental effects in obstructive pulmonary disease. Diuretics are extremely useful in most forms of hypertension (choice D). Because strong vasodilators (choice E) evoke powerful compensatory responses (salt and water retention, tachycardia), drugs like minoxidil are almost always used together with drugs such as diuretics that prevent the compensatory responses.

• Question 278:

  Your patient reports being frightened by heart palpitations and dizziness. Further evaluation reveals that the patient has used a substance heavily, but stopped about 3 days prior to experiencing symptoms, which of the following is the most likely substance used?

  A. alprazolam
  B. cannabis
  C. cocaine
  D. dextroamphetamine
  E. heroin
  A. alprazolam

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  Explanation

  Section: Behavioral Science and Biostatics Benzodiazepine withdrawal is often associated with anxiety symptoms. Cocaine (choice C) may cause a "crash" with anxiety soon after cessation of use, but by day 3 there is more likely to be dysphoria than anxiety. Although withdrawal from other substances (choices B, D, and E) can cause similar anxiety, benzodiazepine withdrawal is the most common cause of this type of symptomatology

• Question 279:

  A 24-year-old construction worker who had four injections of the DPT (diphtheria, pertussis, and tetanus) vaccine in his first year of life and boosters at ages 5 and 19 received a deep laceration while excavating a building's foundation. Which of the following is the preferred treatment?

  A. equine tetanus immune globulin, because it will passively immunize him
  B. human tetanus immune globulin, because it will stimulate his anamnestic response
  C. penicillin, because it will kill the organism
  D. streptomycin, because it will kill the organism
  E. tetanus toxoid, because it will stimulate his anamnestic response
  E. tetanus toxoid, because it will stimulate his anamnestic response

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  Explanation

  Section: Microbiology/Immunology Since there are memory lymphocytes primed by a previous tetanus toxoid injection, booster immunization with tetanus toxoid will lead to rapid production of adequate levels of protective antibody. This is the routine procedure followed by physicians for trauma patients, who have been accinated against tetanus and received booster immunization within the last 57 years. The antibody titer to tetanus toxoid remains at protective levels for 510 years. Aminoglycosides such as streptomycin (choice D) or penicillin (choice C) will not be effective against the spores or vegetative cells of C. tetani. Human tetanus immune globulin will only provide antibodies to tetanus exotoxin for a short time and cannot induce an anamnestic response because an injection of tetanus exotoxin or toxoid is required (choice B). Individuals who have been vaccinated against tetanus require injections of the tetanus toxoid to induce rapid production of adequate levels of protective antibody. Treatment with equine tetanus immunoglobulin, which will only provide antitetanus antibody for a short time, is not the preferred method of treatment for this patient (choice A).

• Question 280:

  A neurology resident is testing the jaw-jerk reflex in a patient by tapping gently on the right masseter muscle and observing elevation of the mandible. What is the location of the neuronal cell bodies of the proprioceptive fibers mediating the jaw-jerk reflex?

  A. mesencephalic trigeminal nucleus
  B. motor trigeminal nucleus
  C. principal (main) trigeminal nucleus
  D. spinal trigeminal nucleus
  E. trigeminal (gasserian) ganglion
  A. mesencephalic trigeminal nucleus

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  Explanation

  Section: Anatomy The jaw-jerk reflex is a monosynaptic (stretch) reflex for the masseter muscle. Proprioceptive fibers from the muscle travel by way of the trigeminal nerve back to their cell bodies in he mesencephalic trigeminal nucleus. Projections from this nucleus synapse on motor neurons of the motor trigeminal nucleus (choice B) which elicits contraction of the masseter muscle. The principal (main) trigeminal nucleus (choice C) receives light touch sensory information from the face, whereas the spinal trigeminal nucleus (choice D) receives pain and temperature sensations. The trigeminal (gasserian) ganglion (choice E) contains the cell bodies of all sensory neurons projecting to the principal (main) and spinal trigeminal nuclei.