USMLE USMLE Certifications USMLE-STEP-1 Questions & Answers

• Question 341:

  Aspecimen of thick, bloody sputum from a hospitalized 80-year-old patient with chronic obstructive pulmonary disease is sent for laboratory analyses. The tests yield heavy growth of a lactose-positive, nonmotile, gram-negative rod with a large gelatinous capsule. Which of the following is this bacterium most likely to be?

  A. E. coli

  B. Enterobacter aerogenes

  C. K. pneumonia

  D. P. aeroginosa

  E. Y. pseudotuberoulosis

  Correct Answer: C

  Section: Microbiology/Immunology

  K. pneumoniae (choice C) is present in the respiratory tract and feces of about 5% of normal individuals. It causes a small proportion (about 1%) of bacterial pneumonias. It is a gram-negative rod with a well-defined capsule that helps to resist phagocytosis by PMNs. It can produce extensive hemorrhagic necrotizing consolidation of the lung in debilitated patients and can cause nosocomial (hospital acquired) infections. Elderly patients with COPD are particularly at risk with this organism. Other enteric (choices A and B) may also produce pneumonia but do not have the well-defined capsule that K. pneumoniae possesses. P. aeruginosa (choice D) is a widely distributed gramnegative bacillus that can also be quite opportunistic for individuals with abnormal host defenses. P. aeruginosa produces pigments that produce bluegreen pus and is especially a problem with cystic fibrosis patients where the organism resides in a well-defined biofilm within the patient. Y. pseudotubeculosis (choice E) resides in domestic farm animals and is the source of human infections.

• Question 342:

  A newborn has a temperature of 103癋. Blood culture grows out gram-positive cocci in chains. Which of the following is the causative agent?

  A. Group A Strep (S. pyogenes)

  B. Group B Strep (S. agalactae)

  C. N. meningitides

  D. Salmonella species

  E. S. pneumoniae

  Correct Answer: B

  Section: Microbiology/Immunology The streptococci were divided into multiple groups based upon different soluble carbohydrates isolated from the cell walls. This CHO was antigenic, and R. Lancefield produced antibodies against each of them. She initially identified Groups AG. Almost 95% of streptococcal infections in humans are caused by Group A, S. pyogenes (choice A). S. agalactiae is a representative member of the Group B streptococcous (choice B). They are members of the normal flora of the female genital tract and an important cause of neonatal sepsis and meningitis. They produce beta (clear zone) hemolysis, like the Group A members. They can be identified as Group B on the basis of the CAMP test (positive) and monoclonal antibody screening tests. It is important to identify and treat such neonatal infections properly to avoid mortality of the small patients. While N. meningitidis (choice C) may affect infants, most cases of meningitis caused by the organism would be in 5-yearold to young adult patients. Salmonella species (choice D) is an inappropriate choice for thisage and disease. S. pneumoniae (choice E) can affect all age groups, but the highest incidence for meningitis is in the elderly population.

• Question 343:

  Autoantibody testing of a patient with an autoimmune disorder reveals high titers of antibodies against the core proteins of small nuclear ribonucleoprotein particles (Smith antigen). Based on this finding, detection of which of the following is the most important initial clinical feature of this autoimmune disorder?

  A. a lesion known as erythema chronicum migrans

  B. a rash in a butterfly distribution on the face

  C. antibody to single-stranded RNA

  D. carditis E. pneumonitis

  Correct Answer: B

  Section: Microbiology/Immunology The autoimmune disease described here by the presence of anti-Sm antibodies is Systemic lupus erythematosus (SLE). This autoimmune disease is initially recognized clinically by a rash on the face with a "wolf"-like shape and thus the name lupus. Such a butterfly rash (over the malar eminences of the face, hence a malar rash) is considered a distinctive manifestation of SLE. The development of erythema chronicum migrans is a pathognomonic feature of Lyme disease (choice A). Detection of antibodies to double-stranded DNA are pathognomonic for SLE (choice C). Arthritis, but not carditis (choice D) or pneumonitis (choice E), occurs frequently.

• Question 344:

  An elderly patient was hospitalized with severe flu. An enveloped virus was isolated from this patient. Which of the following pertains to this RNA virus with a segmented, negative-sense genome?

  A. can have direct viral genome and host cell ribosome interaction

  B. cannot possess a lipid envelope

  C. must contain a DNA-dependent RNA polymerase

  D. must contain an RNA-dependent DNA polymerase

  E. must contain an RNA-dependent RNA polymerase

  Correct Answer: E

  Section: Microbiology/Immunology The patient was hospitalized due to severe influenza symptoms, and an influenza virus was isolated from clinical specimen. Isolation of the etiologic virus would not change the way medical care was being given to the individual. Influenza viruses have an RNA genome comprising of eight segments, each coding for one or more viral components. Influenza viral genomes are also negative-sense, meaning that within the virus particle, there must be an RNA-dependent RNA polymerase to allow the virus growth cycle to make viral mRNA and viral genomic RNA (choice E). There would be no use for a DNA-dependent RNA polymerase (choice C) or an RNA-dependent DNA polymerase (choice D). Since the influenza virus uses the host cell as a factory to produce new viral progeny, host cell ribosomes would be utilized for viral protein production (choice A) but by definition, negative-sense RNA cannot act as mRNA. Therefore, RNA from the virion could not directly interact with host cell ribosomes. Influenza viruses possess a well-defined lipid envelope (choice B), where the HA and NAglycoproteins are found.

• Question 345:

  A 39-year-old man with normal sexual practices saw his doctor, complaining of a mild (38.5°C) but persistent fever. His physician observed pharyngitis and a bilateral conjunctivitis that the patient claimed was recent and painful. Which of the following microbes most likely caused the infection?

  A. adenovirus

  B. C. pneumoniae

  C. N. gonorrhoeae

  D. S. pyogenes

  E. Toxoplasma gondii

  Correct Answer: A

  Section: Microbiology/Immunology Adenoviruses are a frequent cause of infection characterized by a triad of symptoms, namely, pharyngitis, conjunctivitis, and fever. S. pyogenes (choice D) causes 25% of cases of pharyngitis. However, the disease is often accompanied by tonsillitis. The infection is usually associated with fever which is higher than 38.5°C. N. gonorrhoeae (choice C) normally causes gonorrhea. C. pneumoniae (choice B) causes generally primary atypical pneumonia which is not usually associated with conjunctivitis. T. gondii (choice E) in AIDS patients, or other immunosuppressed individuals can cause disseminated disease and encephalitis (choice E).

• Question 346:

  S-pyogenes, Group A, is subdivided into specific antigenic types principally on the immunologic differences in which of the following?

  A. hyaluronic acid capsule

  B. M protein

  C. streptolysin O

  D. streptolysin S

  E. TSST

  Correct Answer: B

  Section: Microbiology/Immunology M protein (choice B) is a major virulence factor of Group AS. pyogenes. M proteins appear as hair-like projections of the cell wall which resist phagocytosis by PMNs. Group A Streptococci are nonvirulent, and antibodies to specific M proteins will protect against reinfection. There are, however, more than 80 types of M protein, with little cross-protection observed. M protein may well have an important role in the pathogenesis of rheumatic fever. Purified streptococcal cell wall membranes induce antibodies that cross-react with human cardiac sarcolemma. Acomponent of the cell wall of selected M types induces antibodies that react with cardiac muscle tissue. The M protein serotypes are used to subdivide Group A streptococci into 80 different serotypes. The other choices (A, C, D, and E) are all virulence factors of Group Astreptococci, but do not form the basis of identifying antigenic different subgroups or serotypes.

• Question 347:

  Several Neisseria species are part of the normal flora (commensals) of the human upper respiratory tract. Which of the following statements accurately describes the significance of these bacteria?

  A. As a part of the normal flora, neisseriae provide a natural immunity in local host defense.

  B. As a part of the respiratory flora, they are the most common cause of acute bronchitis and pneumonia.

  C. Commensal bacteria stimulate a cellmediated immunity (CMI).

  D. Commensal neisseriae in the upper respiratory tract impede phagocytosis by means of lipotechoic acid.

  Correct Answer: A

  Section: Microbiology/Immunology The term normal or usual microbial flora denotes the microorganisms on the skin and mucus membranes of healthy individuals. This resident flora consists of relatively fixed types of organisms that promptly reestablishes itself if disturbed. If normal flora is altered due to use of broad-spectrum antimicrobials, for instance, transient microorganisms may colonize, proliferate, and cause disease. The resident flora are commensals (symbiotic, not harmful; one may benefit from the other). Normal flora may become opportunistic pathogens if they become established in some other part of the human host. Since normal flora microorganisms are antigenic, it has been proposed that normal flora (like these Neisseriae) help to provide a natural immunity in local host defense (choice A). While there is a normal respiratory flora (choice B), most respiratory diseases (bacterial and/or viral) are usually transmitted from the environment or from other people. Bacteria, in general, affect the humeral (antibody) immunity (choice C). Some noncommensal bacteria may stimulate the CMI (mycobacteria,for example). Neisseria pathogens (gonococci and meningococci) possess capsules and produce mucoid colonies (evidence of capsules), while normal flora colonies are opaque and wrinkled, indicating no capsules (choice D). Techoic acids and lipotechoic acid residues are usually associated with gram- positive bacteria.

• Question 348:

  A 3-year-old child with a history of recurrent staphylococcal infections arrives at the emergency department at the local hospital. His neutrophils show normal chemotaxis, degranulation, and phagocytosis. However, intracellular killing of staphylococci by the neutrophils is severely impaired. Myeloperoxidase activity of his neutrophils is normal. The child has no history of streptococcal infection. Which of the following is the most likely disease affecting this patient? Which of the following explains the reason why the child described in question 530 has no history of streptococcal infection?

  A. he has been administered antistreptolysin O

  B. he has been immunized with CRP

  C. he has been injected with pooled human gamma globulin

  D. phagocytosis plays a minor role in the killing of streptococci

  E. streptococci are catalase negative

  Correct Answer: E

  Section: Microbiology/Immunology Streptococcal infections in chronic granulomatous disease are rare, because the defective myeloperoxidase-hydrogen peroxide-halide system can use hydrogen peroxide accumulated by phagocytized streptococci, which are catalase negative and cannot convert hydrogen peroxide to water and oxygen. Administration of pooled gamma globulin (choice C) or antistreptolysin (choice A) is likely to provide only brief, but not chronic, protection against streptococci. C-reactive protein (CRP) is a beta serum globulin which reacts with the C carbohydrate of the cell wall of S. pneumoniae. It is not an antibody and thus will not provide even brief protection against streptococci (choice B). Phagocytosis plays a major, not a minor, role in the killing of streptococci (choice D).

• Question 349:

  A 3-year-old child with a history of recurrent staphylococcal infections arrives at the emergency department at the local hospital. His neutrophils show normal chemotaxis, degranulation, and phagocytosis. However, intracellular killing of staphylococci by the neutrophils is severely impaired. Myeloperoxidase activity of his neutrophils is normal. The child has no history of streptococcal infection. Which of the following is the most likely disease affecting this patient?

  A. chronic granulomatous disease

  B. Graves disease

  C. rheumatoid arthritis

  D. severe combined immunodeficiency disease (SCID)

  E. systemic lupus erythematosus (SLE)

  Correct Answer: A

  Section: Microbiology/Immunology This patient has chronic granulomatous disease. Children with this disease suffer chronic suppurative infections caused most frequently by S. aureus. The cause of chronic granulomatous disease is most commonly an X-linked disorder related to a genetic defect in NADPH oxidase, leading to a deficiency of NADPH oxidase. Neutrophils contain myeloperoxidase, which utilizes hydrogen peroxideand halide ions to produce hypochlorite, which is highly microbicidal. Because of the defective NADPH oxidase, the patient's phagocytes cannot generate sufficient hydrogen peroxide, thus the myeloperoxidase- hydrogen peroxide-halide system cannot function normally. Neutrophils from these patients can phagocytize bacteria and fungi normally, but they cannot kill them because of the defective myeloperoxidase-hydrogen peroxide-halide system. Also, laboratory analysis reveals an absent respiratory burst as demonstrated by a negative nitroblue tetrazolium test in patients with chronic granulomatous disease Severe combined immunodeficiency disease (SCID) is characterized by the absence of T and B cells, absence of thymus, and a lack of lymphocyte proliferative response to mitogens, antigens, and allogeneic cells in vitro (choice D). Systemic lupus erythematosus is an autoimmune disease (type III hypersensitivity or immune-complex disease). Patients with lupus erythematosus produce anti-DNA antibodies (choice E). Rheumatoid arthritis is another autoimmune disease. Patients with this disease produce IgM autoantibodies to the Fc moiety of IgG (choice C). Patients with Graves disease produce autoantibodies to thyroid-stimulating hormone receptors (choice B).

• Question 350:

  The patient's viral infection can best be prevented by which of the following treatments?

  A. administration of human gamma globulin

  B. immunization with HBcAg

  C. immunization with the specific inactivated virus

  D. immunization with the specific subunit vaccine

  E. interferon

  Correct Answer: C

  Section: Microbiology/Immunology Hepatitis virus Ainfection can best be prevented by immunization with the intact inactivated virus. Interferons act only in the early phase of a viral infection, limiting the spread of virus, and are not recommended for long-term protection against viral diseases (choice E). Immunization with a subunit vaccine is used to prevent hepatitis virus B infection, not hepatitis virus Ainfection (choice D). Hepatitis B core antigen (HBcAg) is not effective for vaccination, even against the hepatitis B virus from which it is derived (choice B). Administration of human gamma globulin may provide only passive immunity, which will be of short duration, and thus it is not the best way to prevent hepatitis A viral infection (choice A).