USMLE USMLE Certifications USMLE-STEP-1 Questions & Answers

• Question 411:

  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

  Correct Answer: A

  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 412:

  After extraction of a wisdom tooth, an 18-yearold male student was diagnosed as having subacute bacterial endocarditis. He has a congenital heart disease which has been under control. Which of the following is the most likely organism causing his infection?

  A. Enterococcus faecalis

  B. S. aureus

  C. S. epidermidis

  D. S. pneumonia E. S. viridans

  Correct Answer: E

  Section: Microbiology/Immunology The viridans streptococci include S. mitis, S. mutans, S. salivarius, S. sangus, and others. They make up a major part of oral normal flora and are alpha hemolytic. They reach the bloodstream as a result of trauma and are a principal cause of endocarditis on abnormal heart valves (choice E). They produce large amounts of polysaccharides and contribute to dental caries. S. aureus (choice B) and S. epidermidis (choice C) are usually found on the skin and may be opportunistic, but they are not as significant for endocarditis as the alpha hemolytic streptococci. E. faecalis (choice A) are usually found in the intestinal tract flora and are opportunistic if they escape that location. The major lesion for enterococci is an abscess. S. pneumonia (choice D) is primarily found in lung infections (pneumonias) and are pathogenic primarily due to their polysaccharide capsules which inhibit phagocytosis.

• Question 413:

  An injured firefighter developed a wound infection, and culture of the site indicates a gram-negative rod that is oxidase-positive and produces a bluish-green pigment. The organism was relatively resistant to antibiotics, but susceptible to ticarcillin and tobramycin. The organism is most likely to be identified as which of the following?

  A. E. coli

  B. Klebsiella pneumonia

  C. P. mirabilis

  D. P. aeruginosa

  E. Serratia marcescens

  Correct Answer: D

  Section: Microbiology/Immunology

  P. aeruginosa is a gram-negative, oxidasepositive, aerobic rod that produces a greenblue pigment called pyocyanin. This microorganism has been associated frequently with wound infections in burn patients, and it is the second leading cause of burn infections after S. aureus. P. aeruginosa tends to develop resistance to various antibiotics. However, it may respond to ticarcillin, gentamycin, tobramycin, piperacillin, or azlocillin. E. coli (choice A), K. pneumoniae (choice B), P. mirabilis (choice C), and S. marcescens (choice E) may cause urinary or pulmonary tract infections, but are not considered leading causes of burn infections. Furthermore, these bacteria are oxidase negative and do not produce blue green pigments.

• Question 414:

  The tolerance of facultative anaerobic bacteria, many of which may be normal flora and opportunistic, to superoxide and hydrogen peroxide is due to which of the following?

  A. absence of catalase

  B. cytochrome oxidase

  C. inability to form oxygen

  D. presence of peroxidase

  E. presence of superoxide dismutase and catalase

  Correct Answer: E

  Section: Microbiology/Immunology Superoxide, which is bactericidal, is generated during electron transport and in the autoxidation of hydroquinones, leukoflavins, ferredoxins, and flavoproteins. Superoxide dismutase, however, forms oxygen and hydrogen peroxide from superoxide radicals. When catalase is present, it destroys the bactericidal hydrogen peroxide, because catalase hydrolyzes hydrogen peroxide to water and oxygen. Thus, the presence of both enzymes allows aerobes, facultative anaerobes, andaerotolerant microbes to survive when they grow in the presence of oxygen. Obligate anaerobic bacteria generally lack superoxide dismutase or catalase or both and thus cannot grow in the presence of oxygen. Oxidase is an enzyme that oxidizes a substrate by the addition of oxygen or the removal of hydrogen. It cannot oxidize the bactericidal radical superoxide (choice B). Catalase is an enzyme that converts the bactericidal substance hydrogen peroxide to water and oxygen. It has no effect on the microbiocidal action of superoxide. Bacteria that lack catalase are likely to be suppressed by hydrogen peroxide (choice A). Peroxidase is an enzyme which catalyzes the dehydrogenation of a substrate in the presence of hydrogen peroxide, which acts as hydrogen acceptor and becomes converted into two molecules of water. Thus, possession of peroxidase in a given microbe will allow it to grow in the presence of hydrogen peroxide, but not in the presence of superoxide (choice D). There are no known pathogenic bacteria that cannot form oxygen (choice C).

• Question 415:

  German measles virus (rubella), a common cause of exanthemas in children is best described by which of the following statements?

  A. incubation time is approximately 34 weeks

  B. infections routinely fail to cause an immune response

  C. it can be diagnosed by the presence of Koplik's spots

  D. rubeola and rubella are caused by the same virus

  E. specific antibody in the serum does prevent disease

  Correct Answer: E

  Section: Microbiology/Immunology Rubella virus causes German measles or 3-day measles. Rubella virus is a stable RNA virus classified in the Togavirus group, but is not spread by any insect vector. The route of infection of rubella virus is the respiratory tract, with spread to the lymphatic tissue and then to the blood (viremia). Maternal viremia is followed by infection of the placenta, which leads to congenital rubella. Many organs of the fetus support the multiplication of the virus, which does not seem to destroy the cells, but reduces the rate of growth of the infected cells. Viremia develops in 79 days, not 34 weeks (choice A). Viremia lasts until the appearance of antibody on about day 1315. Rubella virus is an excellent antigen which produces an excellent, and possibly life-long, immune response (choice B). Koplik's spots (choice C) are buccal lesions that are seen in rubeola (measles) infections. Rubeola is a paramyxovirus while rubella virus, as mentioned, is a togavirus (choice D). There is an excellent live-attenuated vaccine for rubella that has been given to most children in the United States, and we have seen the number of cases drop from approximately 70,000 in 1969 to a few hundred today. Obviously, a specific antibody in the serum does prevent disease (choice E).

• Question 416:

  Patients with fungal pneumonias are not isolated from other patients because of which of the following?

  A. These infections are caused by organisms which multiply in lymph nodes.

  B. These infections are difficult to transmit from person to person.

  C. These infections are eliminated quickly by antifungal drugs (ketoconazole).

  D. These infections are naturally resolved in 24 hours.

  E. These infections provoke an effective immune response.

  Correct Answer: B

  Section: Microbiology/Immunology In general, with the exception of some superficial infections, mycoses are not readily transmitted from person to person. Thus, patients with fungal pneumonias are not isolated, and hospital personnel are not generally required to wear masks. Fungal pneumonias are not resolved in 24 hours, they can become chronic, and they need long-term treatment with antifungal drugs (amphotericin B) due to the high percent of cases that relapse (choices C and D). Fungi that cause pneumonias are phagocytized by pulmonary macrophages and multiply in them (choice A). Most of the fungal pneumonias occur in immunosuppressed patients who do not mount an effective immune response (choice E).

• Question 417:

  Which of the following is the best neutrophil and monocyte attractant?

  A. C5a

  B. HLA-A

  C. HLA-B

  D. J chain

  E. variable region of the heavy chain of gG

  Correct Answer: A

  Section: Microbiology/Immunology C5a is a component of complement. Activation of complement by endotoxin, or antigen-antibody complexes produces C5a, which is a powerful neutrophil, monocyte, eosinophil, and basophil attractant. C5a along with C3a are anaphylatoxins which mediate histamine release from mast cells leading to increased vascular permeability and vasodilation. The variable region of the heavy chain of IgG is not known as the best neutrophil or macrophage attractant (choice E). The J moiety of IgM and IgA does not possess chemotactic properties for neutrophils and macrophages (choice D). HLA-A and HLA-B are genes for the human leukocyte antigens (HLA), and they control the synthesis of class I antigens (choices B and C).

• Question 418:

  Which of the following correctly describes Norwalk virus?

  A. contains an RNA genome with a single open reading frame

  B. generally causes a severe chronic gastroenteritis with diarrhea that may persist for several weeks

  C. is a major cause of severe diarrhea in infants in the developing countries

  D. is an important cause of epidemic gastroenteritis in the United States

  E. is diagnosed by isolation and then growth of the virus in tissue culture

  Correct Answer: D

  Section: Microbiology/Immunology Many outbreaks of intestinal disease and diarrhea occur in crowded conditions like cruise ships, where hundreds of tourists become ill in a short time and the cruise must be terminated early so that treatment may be obtained and the ship sanitized. Norwalk virus belongs to the calicivirus classification and is an important cause of outbreaks of vomiting and diarrheal illness in families, communities, and institutions. It is widespread in the United States and is the most important cause of epidemic viral gastroenteritis in adults (choice D). Caliciviruses do have a single-stranded RNA genome which is positive-sense and nonsegmented. It has multiple open reading frames for translation (choice A). The gastroenteritis caused by Norwalk virus has a 2448 hour incubation, rapid onset, and a brief time course (1260 hours), not several weeks (choice B). Rotaviruses are the major cause of severe diarrhea in infants in developing and developed countries (choice C). Reverse transcriptase-- PCR is the most widely used method to diagnose Norwalk and other caliciviruses in outbreaks since the virus cannot be isolated, grown, and identified in diagnostic laboratories (choice E).

• Question 419:

  Aprimary viral isolate from a suspected case of poliomyelitis was inoculated into cell culture, and a dramatic cytopathic effect (CPE) was noted within 24 hours. The isolate was confirmed as poliovirus by neutralization with polyvalent antibody to poliovirus types I, II, and III; however, monospecific antibody to each type failed to block CPE. This finding suggests that the isolated contained which of the following?

  A. hybrid virus of type I and type II poliovirus

  B. mixture of polio and another type of picornavirus mixture

  C. mixture of two types of poliovirus

  D. recombinant of type I and type II viruses

  E. virus that shares a few antigenic determinants with poliovirus

  Correct Answer: C

  Section: Microbiology/Immunology Polioviruses 1, 2, and 3 are enteroviruses that belong to the picornavirus group. Even though wild-type polioviruses have not been detected in North America for many years, both the live-attenuated and killed vaccines are currently used in pediatric vaccinations. It is fairly easy to isolate and identify poliovirus strains. In this case, poliovirus was isolated and neutralized by a polyvalent antibody against all three polio strains, prompting a presumptive diagnosis of poliomyelitis in the patient. A follow-up study using separate monovalent antibodies failed to duplicate the viral identification that was indicated by the trivalent serum product. The most logical reason for this would be that two types of polioviruses were isolated and neutralized by the polyvalent reagent but not the monovalent reagents (choice C). While work is being reported on attempts to create a recombinant virus with 1, 2, and 3 antigen markers (choice D), this has not yet become successful. Neither have any hybrid polio viruses (choice A) been reported. A mixture of poliovirus and another picornavirus (choice B) would not explain the situation described because there is little to no cross reactivity of poliovirus antisera to the other members of the picornavirus group. Choice E would not be appropriate either because no other picornavirus shares only some antigenic determinants with the polioviruses.

• Question 420:

  A 6-year-old girl has been having recurrent pyogenic bacterial infections of the sinus and respiratory tract. The latest one has been caused by N. meningitidis. The results of her diagnostic tests are shown in below figure.

  

  These results are consistent with a diagnosis of which of the following?

  A. chronic granulomatous disease

  B. deficiency in the opsonization of microbes by phagocytes

  C. Graves disease

  D. systemic lupus erythematosus

  E. Wiskott-Aldrich syndrome

  Correct Answer: B

  Section: Microbiology/Immunology The diagnostic tests ordered by the physician are consistent with a deficiency in the C3, C5C8 components of complement. Individuals with these deficiencies have recurrent pyogenic infections, and show enhanced susceptibility to meningococcal infections. All other choices are not consistent with the laboratory findings. Chronic granulomatous disease (choice A) entails a defect in the intracellular killing of microbes by neutrophils. Graves disease (choice C) involves autoantibody production to thyroid-stimulating hormone receptors. Systemic lupus erythematosus (choice D) involves production of antinuclear antibodies (ANA). Wiskott-Aldrich syndrome (choice E) is associated with recurrent pyogenic infections, but is due to an inability of plasma cells to produce IgM against bacterial polysaccharides, and it occurs only in male infants.