USMLE USMLE Certifications USMLE-STEP-1 Questions & Answers

• Question 451:

  A 22-year-old man complains to his family physician of fatigue, night sweats, and a dry unproductive cough. Until the past few months, he had apparently been in good health. Acomplete blood count (CBC) and differential blood count reveal that he is lymphopenic. X-ray examination reveals an interstitial pneumonia. Skin test reactions to a battery of materials are normal. Which of the following should be the next step in evaluating this patient's illness?

  A. CH50 complement assay

  B. chemotaxis assay

  C. identification of the organism that is causing the pneumonia

  D. intracellular killing assay

  E. nitroblue tetrazolium reduction assay

  Correct Answer: C

  Section: Microbiology/Immunology The patient described in the question probably has AIDS. Such individuals will be lymphopenic and have greatly reduced immunity. AIDS victims commonly develop P. carinii pneumonia. The most important evaluative procedure for the patient described is determining the cause of the pneumonia so the problem can be corrected. Identifying the infecting organism may assist in the overall diagnosis. Choices A, B, D, and E involve innate immunity; either complement activity or phagocytic cell functions, and thus, would not be of much diagnostic assistance.

• Question 452:

  A 25-year-old female executive presents with edema, hematuria, proteinuria, and decreased urination indicative of glomerulonephritis. Three weeks ago she had impetigo on her chest which resolved without treatment. Which of the following organisms is the most likely cause of her renal disease?

  A. E. coli

  B. S. aureus

  C. S. saprophyticus

  D. Streptococcus agalactiae

  E. S. pyogenes

  Correct Answer: E

  Section: Microbiology/Immunology

  Acute glomerulonephritis appears as a complication after pharyngeal or skin infections with Group A S.

  pyogenes. Patients with poststreptococcal glomerulonephritis develop high levels of antibodies against

  streptolysin O of S. pyogenes. While S. aureus (choice B) may also cause impetigo, it is seldom, if ever,

  found to be the cause of postinfectious glomerulonephritis. None of the other organisms (choices A, C, or

  D) have been identified with this postinfectious complication either.

• Question 453:

  A 42-year-old woman had a catheter placed in her urethra. One week later she is experiencing suprapubic flank pain with urinary urgency and frequency. She has also chills and fever. After examining the patient and evaluating the sediment of her centrifuged urine, she is informed by her physician that she has acute ascending pyelonephritis. If the diagnosis is accurate and the patient's urine is cultured, which of the following organisms will most likely be isolated?

  A. Clostridium difficile

  B. Escherichia coli with pili

  C. Pseudomonas aeruginosa

  D. Staphylococcus aureus

  E. streptococci--group B

  Correct Answer: B

  Section: Microbiology/Immunology Piliated strains of E. coli move up the urethra to infect the bladder and kidney. Infections of the kidney cause pyelonephritis. The vast majority of cases of bacterial pyelonephritis, cystitis, and other urinary tract infections are caused by E. coli. Introduction of catheters into the urethra has been associated with the occurrence of urinary tract infections. C. difficile is the cause of pseudomembranous colitis (choice A). S. aureus is usually the cause of boils, skin sepsis, postoperative wound infections, scalded skin syndrome, food-borne infection, septicemia, endocarditis, toxic shock syndrome, osteomyelitis, and pneumonia (choice D). P. aeruginosa is usually the cause of infections of skin and burns. It is also the major pathogen in cystic fibrosis, and can cause urinary infections, but not as commonly as E. coli (choice C). Group B streptococci (choice E) are usual vaginal flora, not pathogenic to the host but may be opportunistic to newborns.

• Question 454:

  While on a wilderness vacation, a banker develops extensive skin lesions noted for their varying degrees of erythema, edema, and vesiculations. His physician tells him that these lesions are due to delayed type hypersensitivity. If this is actually the case, which of the following statements is accurate?

  A. Delayed-type hypersensitivity can be transferred passively to volunteers by sensitized lymphocytes.

  B. Delayed-type hypersensitivity is suppressed by antihistaminic drugs.

  C. This allergy does not cause tissue damage.

  D. This allergy is due to IgE absorbed on mast cells.

  E. This type of allergy usually occurs after inhalation of grass pollens.

  Correct Answer: A

  Section: Microbiology/Immunology Delayed-type hypersensitivity is mediated by helper CD4 lymphocytes, not antibody (choice D). Thus it can only be transferred by sensitized CD4 helper lymphocytes. Sensitization occurs via such substances as poison ivy, poison oak, some cosmetics, topically applied sulfonamides, or other drugs and simple chemicals, such as nickel, formaldehyde, and others, but not inhalation of grass pollen (choice E). Part of the tissue destruction seen in tuberculosis is due to delayed-type hypersensitivity, so this type of hypersensitivity reaction may lead to tissue damage (choice C). Administration of antihistaminic drugs, epinephrine, or cromolyn sodium is used for the treatment of anaphylactic reactions resulting from type I hypersensitivity, but not delayed-type hypersensitivity (choice B).

• Question 455:

  A medical student has been immunized with hepatitis B virus (HBV) recombinant vaccine. The curve in below figure represents the production of protective antibodies to the viral component present in the recombinant vaccine. Which of the following is the known viral component of the HBV vaccine?

  

  A. nucleocapsid proteins of HBV

  B. RNA genome of HBV

  C. viral core antigen (HBcAg)

  D. viral e antigen (HBeAg)

  E. viral surface antigen (HBsAg)

  Correct Answer: E

  Section: Microbiology/Immunology The envelope of hepatitis B virus contains an antigen known as HBsAg (hepatitis B surface antigen). This antigen is of importance because production of antibodies to HBsAg indicates immunity against hepatitis B virus, which resulted either from infection or vaccination with HBsAg. Treatment of HBV with a nonionic detergent removes the envelope and produces the viral core, which contains the hepatitis B core antigen (HBcAg). Antibodies to HBcAg are not protective (choice C). Treatment of the viral core with strong detergents results in the release of a soluble core antigen called hepatitis virus antigen e (HBeAg). Production of antibody to HBeAg signals active disease during which the patient is infectious (choice D). The DNA genome (choice B), or the nucleocapsid proteins of HBV (choice A) are not protective.

• Question 456:

  A 20-year-old woman with generally normal health and immune capacity visited her doctor complaining of 2 weeks of fever, sore throat, and cough. She had recently seen bloodstreaked sputum and had noticeable chest pain. X-rays showed only a moderate amount of pulmonary consolidation, while a cold agglutinin titer of 1:256 was reported. Which organism listed below is the most likely etiological agent?

  A. Chlamydia pneunoniae

  B. Coxiella burnetii

  C. Legionella pneumophila

  D. Mycoplasma pneumonia

  E. Pneumocystis carinii

  Correct Answer: D

  Section: Microbiology/Immunology "Walking" primary atypical pneumonia caused by M. pneumoniae can lead to formation of IgM antibodies called cold agglutinins, because they agglutinate type O human erythrocytes at 4°C. L. pneumophila (choice C), C. burnetii (choice B), P. carinii (choice E), and C. pneumoniae (choice A) may produce symptoms of pneumonia similar to those induced by M. pneumoniae, but normally these microbes are not associated with cold agglutinin formation.

• Question 457:

  Which of the following can most likely restore the immunological competency of this 5-month-old baby?

  A. administration of purine nucleoside phosphorylase

  B. B-cell supplementation

  C. compatible bone marrow transplantation

  D. injection of adenosine deaminase

  E. T-cell treatment

  Correct Answer: C

  Section: Microbiology/Immunology In patients with combined B- and T-cell immunodeficiencies, compatible bone marrow transplantation, from an HLA-identical sibling, can restore immunological competence for bacteria, viruses, fungi, and protozoa. Administration of adenosine deaminase or purine nucleoside phosphorylase may sometimes reduce the number and severity of microbial infections by a slight, temporary stimulation of the production of B and T cells, but it does not restore immunity to microorganisms (choices Aand D). Treatment with compatible B or T cells may provide very brief humoral or cellular immunity (choices Band E).

• Question 458:

  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)

  Correct Answer: E

  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 459:

  A 45-year-old man was admitted to the hospital because of fever, shortness of breath, and weight loss with edema in the lower limbs. Examination revealed cardiomegaly and arrhythmias, and a diagnosis of myocarditis was made. Bacterial blood cultures were negative. Which of the following organisms is most likely responsible for this illness?

  A. adenovirus

  B. coxsackie group A virus

  C. coxsackie group B virus

  D. mumps virus

  E. rhinovirus

  Correct Answer: C

  Section: Microbiology/Immunology Myocarditis is most commonly caused by Coxsackie group B viruses, and may be preceded by gastrointestinal or respiratory symptoms. The mumps virus (choice D), coronaviruses (choice B), adenovirus (choice A), and rhinovirus (choice E) may be associated with influenza like, or gastrointestinal symptoms, but they are not common causes of myocarditis.

• Question 460:

  A 37-year-old man was immunologically competent but presented to his physician with malaise, fever, lethargy, and back pain. Examination also revealed vertebral osteomyelitis, splenomegaly, and hepatomegaly. He was a PPD-negative farm worker who consumed unpasteurized goat milk. Which of the organisms listed below is the mostlikely etiological agent?

  A. Actinomyces israelii

  B. Aspergillus fumigates

  C. Brucella melitensis

  D. Candida albicans

  E. Mycobacterium tuberculosis

  Correct Answer: C