USMLE USMLE Certifications USMLE-STEP-1 Questions & Answers

• Question 361:

  A 25-year-old male patient comes to your office experiencing inability to swallow and speech difficulty. The patient was in perfect health prior to the consumption of home-canned green beans. Which of the following is the most likely method of treatment?

  A. administration of staphylococcal enterotoxin antiserum

  B. administration of trivalent botulinum antitoxin

  C. immunization with S. aureus enterotoxin toxoid

  D. penicillin administration

  E. placement of the patient in a hyperbaric oxygen chamber

  Correct Answer: B

  Section: Microbiology/Immunology The patient described in this question is showing the typical symptoms of botulism, which is commonly caused by types A, B, or E C. botulinum toxin. Therefore, early administration of potent botulinum antitoxin- containing antibodies to toxins A, B, and E constitutes the most appropriate type of treatment. Administration of staphylococcal enterotoxin antiserum could have been helpful if the atient was suffering from staphylococcal food poisoning, but he is not (choice A). Immunization with staphylococcal enterotoxin toxoid (choice C), administration of penicillin (choice A), or placing this patient in a hyperbaric chamber (choice E) are all useless.

• Question 362:

  A 37-year-old male patient presents with a necrotizing skin lesion on the trunk of the body subsequently identified as Blastomyces dermatiditis. Which is the most likely route of entry for this organism?

  A. contaminated water

  B. illicit drug use

  C. lung inhalation

  D. skin prick

  E. spoiled food

  Correct Answer: C

  Section: Microbiology/Immunology

  B. dermatiditis is a dimorphic fungus, producing hyphae and conidia at room temperature but large, singly budding yeast cells at 37°C. Blastomycosis is a chr onic infection with granulomatous and suppurative lesions that is initiated in the lungs. Inhalation (choice C), therefore, would be the mechanism of entry for this organism. It can disseminate to any organ, but the skin and bones appear to be the most common site. Interestingly, while B. dermatiditis is endemic for humans and dogs in the United States, it is not directly transmitted from either of these sources. It is thought to be derived from the environment, perhaps with some factor associated with rivers. It can be treated with amphoteracin B. No reports of transmission have been reported from consuming contaminated water (choice A), drug use (choice B), skin penetration (choice D), or ingestion of food (choice E), spoiled or otherwise.

• Question 363:

  Two days following surgery to repair a defective valve, a patient developed an acute infection caused by a penicillin-resistant strain of S. aureus. below figure shows the penicillin molecule. Which numbered bond in the structure is the site of action of penicillinase?

  

  A. 1

  B. 2

  C. 3

  D. 4

  E. 5

  Correct Answer: B

  Section: Microbiology/Immunology An important part of the penicillin molecule is the beta-lactam group. This group is composed of two carbon and two hydrogen atoms at the top of the group. The bottom part of the group contains one carbon, which is linked to one nitrogen atom and one oxygen atom. Penicillinase breaks the bond between the bottom carbon and nitrogen atom, and destroys the antibacterial activity of penicillin, along with that of cephalosporin, which also possesses a beta-lactam group.

• Question 364:

  Poliovirus type 2 has been isolated from the stool of a 55-year-old patient who has experienced malaise, fever, nausea, and vomiting. An infant grandchild was vaccinated about 3 weeks prior to onset of the disease. What should the laboratory do to determine whether the isolated virus is related to the vaccine strain or a wild-type virus?

  A. determine the cytopathic effects of the virus

  B. do neutralization studies using the infant's serum

  C. do oligonucleotide mapping of the wildtype and vaccine strains

  D. inoculate the virus into mice to determine whether it kills them

  E. stain the virus with fluorescent antibody

  Correct Answer: C

  Section: Microbiology/Immunology To determine whether the poliovirus is a wild-type virus or if it is related to that used for vaccination, it will be necessary to prepare oligonucleotide maps of the isolated virus and compare them to those of the wild-type and vaccine strains. Many viruses may kill mice; thus, one cannot determine whether poliovirus strains are related or unrelated by mouse lethality studies (choice D). Cytopathology cannot be used for definitive diagnosis of poliovirus strains or other types of viruses (choice A). Viral neutralization assays using the grandchild's serum will only indicate whether the grandchild has been exposed to and vaccinated for the polioviruses in question, but will not establish whether the poliovirus isolated from the stool of the grandfather is related to the poliovirus used to vaccinate his grandchild (choice B). Staining of the virus with fluorescent antibody may detect the presence of poliovirus, but will not establish whether the poliovirus is a wild-type virus or if it is related to that used for vaccination (choice E).

• Question 365:

  Ajudge has a fungal infection known as "Tinea cruris." Which one of the following statements concerning this infection is accurate?

  A. It cannot be transmitted from person to person.

  B. It involves a fungus that produces aflatoxins.

  C. It is associated with athlete's foot.

  D. It is caused by a fungus that forms tuberculate macroconidia.

  E. It occurs primarily in immunocompromised individuals.

  Correct Answer: C

  Section: Microbiology/Immunology Tinea cruris or as commonly known "Jock itch" is typically found in young men, especially in macerated or occluded areas. It is also associated with tinea pedis (athlete's foot) and the causative agent (T. rubrum or

  T. mentagrophytes) may be easily seen in skin scrappings. Tinea cruris occurs in ealthy and immunosuppressed persons (choice E). Histoplasma capsulatum, the causative agent of histoplamosis, forms septate hyphae with tuberculate macroconidia (choice D). In contrast to other fungal infections, tinea cruris is transmitted from person to person (choice A). Aspergillus flavus, not T. rubrum or T. mentagrophytes, produces aflatoxins, which can cause liver damage (choice B).

• Question 366:

  Judging from the graph in below figure

  

  which microorganism is likely to be the causative agent of food poisoning characterized by diarrhea and severe vomiting related to heat-stable enterotoxins?

  A. C. jejuni

  B. Salmonella typhimurium

  C. S. aureus

  D. Yersinia enterocolitica

  E. V. parahaemolyticus

  Correct Answer: C

  Section: Microbiology/Immunology The short incubation of 4 hours indicates staphylococcal food poisoning. This is a situation arising from the ingestion of preformed staphylococcal enterotoxin, which induces symptoms within 16 hours following consumption of contaminated food, including diarrhea, abdominal cramps, and severe vomiting. These symptoms last for 612 hours, and complete recovery usually occurs in less than 1 day. The incubation periods for S. typhimurium (choice B), V. parahaemolyticus (choice D),Y. enterocolitica (choice E), and C. jejuni (choice A) are 812, 2496, 2448, and 72168 hours, respectively. These longer incubation periods in comparison to the incubation period of staphylococcal food intoxication are due to the need for these bacteria to invade the human intestinal tract, and then multiply and form the toxins responsible for the infective form of food poisoning.

• Question 367:

  A retired policeman who received a kidney transplant developed a generalized infection by an enveloped double-stranded DNA virus. This organism formed owl's eye inclusions in cells found in urinary sediments. Which of the following viruses is the most likely etiological agent? Which of the following approaches most likely might have prevented the infection of the policeman?

  A. administration of gamma interferon

  B. injection of alpha interferon

  C. treatment with methisazone

  D. treatment with zanamivir

  E. viral screening of donor and recipient

  Correct Answer: E

  Section: Microbiology/Immunology In this case, prevention of infection most likely involves careful prescreening of the kidney donor and the recipient for CMV presence by antibody detection, isolation, or PCR. For treatment, ganciclovir is moderately effective for CMV pneumonia and retinitis in AIDS patients. Alpha interferon is used for patients with chronic hepatitis caused by B or C virus. However, only some genotypes appear to be susceptible (choice B). Gamma interferon, like alpha, has been shown to have viral replicative inhibitory activity, and also to be able to activate macrophages as well as other immune cells (choice A). Zanamivir inhibits the release of influenza virus from infected cells and thus limits the infection (choice D). Methisazone inhibits protein synthesis of vaccinia and smallpox viruses (choice C).

• Question 368:

  A retired policeman who received a kidney transplant developed a generalized infection by an enveloped double-stranded DNA virus. This organism formed owl's eye inclusions in cells found in urinary sediments. Which of the following viruses is the most likely etiological agent?

  A. adenovirus

  B. coronavirus

  C. cytomegalovirus

  D. papillomavirus

  E. variola virus

  Correct Answer: C

  Section: Microbiology/Immunology Cytomegalovirus (CMV) is an enveloped, double-stranded DNA virus, which causes formation of giant cells with intranuclear inclusion bodies that have an oval owl's eye shape. CMV can cause a generalized infection, which is associated with immunosuppression. This can occur with organ transplantation or AIDS and result in central nervous system involvement, pneumonia, retinitis, colitis, and organ rejection. Adenoviruses (choice A) are also double-stranded DNA viruses but do not have an envelope. Coronaviruses (choice B) contain an RNA genome and do have an envelope. Papillomaviruses (choice D) contain doublestranded DNA and have no envelope. Variola virus (choice E) is a poxvirus, having doublestranded DNA. The largest virus group by size, poxviruses, does contain an "envelope" but it is constructed of multiple membranes closely attached to the virions, unlike lipid envelopes seen in herpes viruses, for example.

• Question 369:

  A high school student while on a field trip encounters poison ivy and develops a severe contact dermatitis,

  which is classified as type IV hypersensitivity. Contact dermatitis is differentiated from type III

  hypersensitivity by which of the following facts?

  Which of the following mechanisms is responsible for the type IV hypersensitivity reaction in the high

  school student?

  A. expression of antigen epitopes on B cells

  B. expression of allergen epitopes on CD8+ T cells

  C. IgE reaction with mast cells

  D. reaction of allergen with the Ti and CD4 markers of B cells

  E. T helper cell reaction with the antigen epitope

  Correct Answer: E

  Section: Microbiology/Immunology The major event associated with IV hypersensitivity is a helper T-cell reaction with the antigen epitope and class II antigen via Ti and CD4 respectively. IgE reacts with mast cells in type I hypersensitivity (choice C). The major event associated with type IV allergy has nothing to do with the expression of antigen epitopes on B cells (choice B), the reaction of allergen with the nonexistent Ti and CD4 markers of B cells (choice D), or the expression of antigen epitopes on CD8+ of T cells (choice A).

• Question 370:

  A high school student while on a field trip encounters poison ivy and develops a severe contact dermatitis, which is classified as type IV hypersensitivity. Contact dermatitis is differentiated from type III hypersensitivity by which of the following facts?

  A. It is associated with cell bound antigens of human erythrocytes.

  B. It is associated with cytotoxic reactions.

  C. It is encountered in atopic individuals.

  D. It is mediated by antigen-antibody complexes.

  E. It is mediated by cellular immunity.

  Correct Answer: E

  Section: Microbiology/Immunology Type IV hypersensitivity, such as contact dermatitis, is a delayed-type allergic reaction that is differentiated from immediate type III hypersensitivity by the fact that it is mediated by a T-cell response. Type III, II, or I allergic reactions are mediated by humoral antibody. Atopic individuals exhibit type I hypersensitivity (choice C). Type II allergic reactions involve production of antibody (IgG, IgM) to cell-bound antigens of erythrocytes. Transfusion reactions are good examples of type II hypersensitivity (choice A). Cytotoxic reactions are type II allergic reactions (choice B). Immune complex reactions represent type III hypersensitivity. Here antigen-antibody (IgG or IgM) complexes accumulate on blood vessels or kidney glomeruli and cause tissue damage. Examples of type III allergy is the Arthus reaction or serum sickness (choice D).