USMLE USMLE Certifications USMLE-STEP-1 Questions & Answers

• Question 391:

  A 2-year-old boy who has not been vaccinated against any germ is experiencing severe spasms in the muscles of the jaw and face. These spasms do not allow the child to open his mouth. Laboratory tests established that a bacterial toxin was responsible for this boy's symptoms. The toxin involved was most likely produced by which of the following organisms?

  A. C. botulinum

  B. C. perfringens

  C. C. tetani

  D. C. diphtheria

  E. V. cholerae

  Correct Answer: C

  Section: Microbiology/Immunology Tetanus toxin produced by C. tetani is a protease that often first affects the masseter muscles. Patients so affected cannot open their mouths and have what is called trismus. Trismus provides an explanation for the term lockjaw used to describe tetanus. Cholera toxin (choice E--V. cholerae) causes fluid and electrolyte loss that leads to severe diarrhea. The clostridial alpha toxin (choice B--C. perfringens) kills cells and produces necrosis. Diphtheria toxin (choice D--C. diphtheriae) is an inhibitor of protein synthesis affecting heart, kidney, and other cells. Protein synthesis is inhibited because diphtheria exotoxin catalyzes the polyADP-ribosylation of eukaryotic elongation factor 2 (eEF2). Botulinum toxin (choice A) cleaves the proteins involved in the release of acetylcholine. This leads to aralysis of ocular, pharyngeal, and respiratory muscles.

• Question 392:

  Gene therapy has taken an approach of expressing TNF in tumor-infiltrating lymphocytes (TIL). It is hoped that this approach will induce which of the following events?

  A. allow the TIL to return to the tumor and produce locally high levels of TNF

  B. generate high serum levels of endotoxin

  C. lyse tumor cells due to retrovirus infection

  D. stimulate natural killer (NK) cell activity

  E. stimulate T-cell proliferation

  Correct Answer: A

  Section: Microbiology/Immunology TNF is cytotoxic to certain tumor cells. Recent experiments in gene therapy have taken the approach of expressing TNF in TIL, with the hope of allowing the TIL to return to the tumor and produce a high concentration of TNF and kill the tumor cells. TNF is a mediator of endotoxin- induced shock and is involved in inflammation, but does not generate high levels of endotoxin (choice B). There is no evidence to support that TNF stimulates T-cell proliferation (choice E). TNF is cytotoxic to certain tumor cells, but it does not lyse tumor cells by retroviral participation (choice C). NK cells can destroy tumor cells; however, they cannot be stimulated by TNF (choice D).

• Question 393:

  An X-linked recessive B-cell immune deficiency associated with a defect in the tyrosine kinase gene and low levels of all antibody classes is most likely to present with repeated infections involving which of the following?

  A. extracellular bacteria

  B. fungi

  C. intracellular bacteria

  D. P. carinii

  E. viruses

  Correct Answer: A

  Section: Microbiology/Immunology Individuals who have been diagnosed as Xlinked agammaglobulinemic lack B lymphocytes and are not able to produce immunoglobulins. The lack of immunoglobulins in X-linked agammaglobulinemia renders individuals susceptible to a succession of infectious diseases caused by extracellular bacteria. These infections may be partially controlled by the injection of specific gamma globulin as a supportive therapy. Xlinked agammaglobulinemic patients have T cells that are the key players in cell- mediated immunity associated with graft rejection, intracellular parasites (choice C), viruses (choice E), and fungi (choice B).

  P. carinii (choice D) is now considered a fungus, causing infections in immunocompromised patients, such as those with AIDS.

• Question 394:

  A 75-year-old lady is slowly developing a chronic suppurative cervicofacial lesion, thought to be actinomycosis. Which of the following is most likely to promote the progress of this infection?

  A. high levels of oxygen in tissues

  B. presence of a foreign body at the site of infection

  C. production of exotoxin by A. israelii

  D. production of leukocidin by A. israelii

  E. production of pili by A. israelii

  Correct Answer: B

  Section: Microbiology/Immunology

  A. israelii is an opportunistic microorganism that does not produce any known virulence factor. It seems that its pathogenesis is related to the ability of this organism to survive the host's inflammatory responses. Devitalized tissues, presence of foreign bodies, broken mucous membranes, and immunosuppression promote development of actinomycosis. A. israelii is an anaerobic organism. Thus high levels of oxygen will kill this microbe (choice A). This bacterium does not produce leukocidin (choice D), exotoxin (choice C), or pili (choice E).

• Question 395:

  Sputum from a patient with a respiratory infection reveals bacteria that resist engulfment by phagocytic WBCs. Which of the following factors is most likely responsible for this virulence?

  A. A capsule

  B. fimbriae

  C. flagella

  D. lipid A

  E. peptidoglycan

  Correct Answer: A

  Section: Microbiology/Immunology Capsules (choice A) are usually made of polysaccharides and allow bacteria to resist phagocytosis by PMNs and macrophages. Since the capsule contributes to the invasiveness of pathogenic bacteria, it is considered a virulence factor. If the host produces an antibody against the capsule materials, phagocytosis and destruction of the organism can occur. Fimbriae (choice B) are also called pili and attach to cell surfaces to promote microbial colonization. While some bacteria (N. gonorrhoeae, for instance) produce pili and are able to resist phagocytosis if firmly attached, fewer kinds of bacteria produce pili than those capable of producing capsules. Flagella (choice C) are organelles of locomotion and do not resist phagocytosis. Lipid A(choice D) is a component part of endotoxin from gram- negative cell walls. It is considered to be the toxic component of endotoxin, capable of causing inflammation, but does not resist phagocytosis. Peptidoglycan (choice E) is part of gram-positive cell walls. Peptidoglycans do not resist phagocytosis directly but do contain techoic acid residues that allow bacteria to adhere to cells.

• Question 396:

  A similar virus was isolated from several students in a day-care school that experienced a respiratory disease outbreak. This virus passes through a 50 nm filter, grows best in human diploid fibroblasts at 33° C, is not inactivated by chloroform, is inactivated at pH 3, and is inhibited by compounds that interact with the interior of grooves on the viral surface that attach to cell receptor molecules. Which of the following is the most likely virus based upon its identifying characteristics?

  A. coronaviruses

  B. Influenza virus B

  C. respiratory syncytial virus

  D. rhinovirus

  E. rubella virus

  Correct Answer: D

  Section: Microbiology/Immunology Rhinoviruses (choice D) are common cold viruses that belong to the picornavirus group. They are small (2830 nanometers), have a positive-sense linear RNA genome, and no lipid envelope. The viral receptor is located in a groove or canyon in the capsid coat where the cellular receptor (ICAM-1) attaches in the initial cell infection process. The incubation period is brief (12 days), allowing many infections to occur in a crowded or closed living situation. Coronaviruses (choice A) are also common cold viruses but larger than 50 nm and has a lipid envelope. Influenza viruses (choice B) are also larger than 50 nm and possess lipid envelopes. RSV (choice C) and rubella virus (choice E) are similar, being larger than 50 nm with lipid envelopes.

• Question 397:

  A 78-year-old patient with an episode of acute urinary retention was catheterized. Three days later, he developed fever and suprapubic pain. Culture of the urine revealed a thin film of bacterial growth over the entire blood agar plate, and the urease test was positive. Which of the following is the most likely organism to cause this infection?

  A. E. faecalis

  B. E. coli

  C. H. pylori

  D. Morganella morganii

  E. P. mirabilis

  Correct Answer: E

  Section: Microbiology/Immunology Proteus species (choice E) produce disease in humans only when the bacteria leave the intestinal tract. P. mirabilis routinely causes urinary tract infections and produces urease. In such urinary tract infections, the urine becomes alkaline, promoting stone production. The organism is rapidly mobile, resulting in swarming on agar plates and may be a virulence factor in the urinary tract invasion. Proteus species are resistant to many antimicrobials. E. coli (choice B) is urease negative, and while motile, does not swarm on agar. H. pylori (choice C) is motile and a strong producer of urease, which is used to modify gastric mucus for more efficient growth in the intestinal tract. Gastritis and peptic ulcers are produced by H. pylori. Morganella morganii (choice D) is similar to proteus but observed only in occasional infections. E. faecalis (choice B) are gram-positive cocci which are urease negative and form colonies on blood agar, not swarming.

• Question 398:

  A 27-year-old man was treated with penicillin for gonorrhea. Thirty-five days later he was reinfected with the same germ, and his physician administered an intramuscular dose of penicillin. Two minutes following the injection of penicillin, the patient experienced hypotension and shock and became unconscious. This reaction was most likely mediated by which of the following?

  A. activation of the alternate complement pathway

  B. activation of the classical complement

  C. IgD

  D. IgE

  E. IgG

  Correct Answer: D

  Section: Microbiology/Immunology Anaphylaxis triggered by penicillin is an immediate hypersensitivity reaction, which is typically mediated by IgE antibodies. IgE antibodies bind to specific Fc receptors on the surface of mast cells and basophils. Upon cross-linking of the IgE antibodies with their specific antigen (penicillin in this case), mast cells and basophils release histamine within minutes along with other pharmacologically active shock mediators which produce the characteristic symptoms of anaphylaxis. Activation of either the classical (choice B) or the alternate complement pathway (choice A) does not play any meaningful role in anaphylaxis. IgD (choice C) and IgG (choice E) are not involved in anaphylactic reactions.

• Question 399:

  Which of the following immunoglobulins is responsible for passing the placental barrier and attacking the Rh+ fetal red blood cells in hemolytic disease of the newborn (erythroblastosis fetalis)?

  A. IgA

  B. IgD

  C. IgE

  D. IgG

  E. IgM

  Correct Answer: D

  Section: Microbiology/Immunology There are five known classes of immunoglobulins: IgG, IgA, IgM, IgD, and IgE. IgG is the major immunoglobulin that is found in human serum and the only one that has been shown to pass the placental barrier in humans. IgM possesses higher agglutinating and complement-fixing capacity than IgG. IgM has a molecular weight of 900,000. Carbohydrates constitute 711% of the total weight of IgM (choice E). IgD constitutes a minor portion of serum immunoglobulins (1%). It contains higher amounts of carbohydrate (13%) than the other immunoglobulins, but it is an important B-cell receptor. No other biological functions have been described for IgD (choice B). IgE is the immunoglobulin that has been associated with anaphylactic hypersensitivity. IgE has a molecular weight of 190,000 200,000, contains 1112% carbohydrate, and constitutes 0.002% of the total serum immunoglobulin (choice C). IgA is the major immunoglobulin of extracellular secretions. It has a molecular weight of 160,000440,000, has modest agglutinating capacity, and its carbohydrate content is two to three times higher (7.5%) than that of IgG (choice A).

• Question 400:

  Within hours after being bitten by a cat trapped in a flood drain, a member of the rescue team develops redness, swelling, and intense pain at the site of the bite. Laboratory results indicate the presence of mixed flora in the wound, including a gram-negative rod that is frequently associated with cat and dog bites and that often leads to complications such as osteomyelitis if untreated. Which of the following is most likely to be this organism?

  A. B. canis

  B. C. coli

  C. Francisella tularensis

  D. Pasteurella multocida

  E. P. aeruginosa

  Correct Answer: D

  Section: Microbiology/Immunology Pasteurella species are primarily animal pathogens, but they can produce a range of human diseases. All yersiniae and franciseliae were formerly classified under this genus. P. multocida (choice D) occurs worldwide in the respiratory and GI tracts of many domestic and wild animals. It is the most common organism in humans inflicted by bites from cats and dogs. It may produce human infections in many systems and may, at times, even be part of the normal/usual human flora. Ahistory of animal bite, pain and pus drainage at the bite site, gram-negative bipolar rods, and culture of the organism would confirm the diagnosis. B. canis (choice A) may cause mild reticuloendothelial disease in humans. F. tularensis (choice C) can be transmitted by direct contact with infected animal tissues or by inhalation or ingestion, seldom if ever by bites. C. coli (choice B), along with C. jejuni, are common human pathogens, causing mainly enteric infections. P. aeruginosa (choice E) is widely distributed in nature and in moist environments in hospitals. It can cause opportunistic diseases in immunocompromised humans. Such infections may present with bluegreen pus and are difficult to treat with antimicrobials.