USMLE USMLE Certifications USMLE-STEP-1 Questions & Answers

• Question 381:

  A 25-year-old woman develops fever, sore throat, headache, and generalized lymphadenopathy, a macular non-pruritic rash over the trunk and limbs, flat papular lesions on her inner thighs. Initial serological tests are positive for Treponema species. Which of the following represents the diagnosis for this condition?

  A. condylomata lata

  B. primary syphilis

  C. secondary syphilis

  D. tertiary syphilis

  E. yaws

  Correct Answer: C

  Section: Microbiology/Immunology Natural infections with T. pallidum are limited to the human host, usually by sexual contact. The infectious lesion is on the skin or mucus membranes of the genitalia. Spirochetes multiply locally at the site of entry and spread to the local lymph nodes and the blood stream. Several weeks later, a papule develops at the site of infection and develops into a lesion (hard chancre). This primary lesion (choice B) "heals" spontaneously, but "secondary" lesions appear 23 months later. These include a red maculopapular rash and moist, pale papules (condylomas) in the genital region, axillas and mouth (choice C). Syphilitic meningitis, chorioretinitis, hepatitis, nephritis, or periostitis may occur. Both primary and secondary lesions are filled with infectious spirochetes. The infection goes subclinical. About 30% of cases cure spontaneously, while another 30% remain latent. In the rest, the disease progresses to the tertiary (choice D) stage characterized by the development of granulatomatous lesions (gumma) in the skin, bones, and liver. Degenerative changes can occur in the CNS or cardiovascular system. Spirochetes are rare at this stage. T. pallidum cannot be cultured, and the disease is usually confirmed by serological tests. Penicillin remains the treatment of choice, but prolonged follow-up is essential. Yaws (choice E) is a disease that is usually seen as ulcerating papules on the skin of arms or legs and is caused by a variant of T. pallidum. Direct contact is the usual transmission mechanism. Condylomata lata (choice A) refer to eroded papules that fuse together and are highly infectious during secondary syphilis.

• Question 382:

  A 3-year-old child presents at the physician's office with symptoms of coryza, conjunctivitis, low- grade fever, and Koplik's spots. The causative agent of this disease belongs to which group of viruses?

  A. adenovirus

  B. herpesvirus

  C. orthomyxovirus

  D. paramyxovirus

  E. picornavirus

  Correct Answer: D

  Section: Microbiology/Immunology Measles or rubeola belongs to the paramyxovirus group (choice D). Measles is an acute, highly infectious disease characterized by fever, respiratory symptoms, and a maculopapular rash. Complications are common and serious. Humans are the only natural host for measles virus, which is spread by respiratory secretions. The virus grows first in the respiratory tract and then to the local lymphatics. Koplik's spots can be observed in the buccal mucosa. The virus replicates in the RE system and is then spread to the skin and other epithelial surfaces. The incubation period can be up to 3 weeks and the rash can last up to 2 weeks (hard measles). The rash appears to be a T-cell interaction with virus-infected cells. The main complication involves the CNS and lungs. Arare complication is SSPE. An excellent live-attenuated virus vaccine has been widely used and has decreased measles presence in the population greatly. Immunity (natural or artificially acquired) is lifelong. Adenovirus (choice A) is most often seen as respiratory infections, although significant eye and enteric diseases are caused by different strains of adenovirus. Herpesvirus (choice B) produces crops of vesicular lesions, not a maculopapular rash. Othromyxovirus (choice C) is influenza, which has no rash presentations. Picornaviruses (choice E) consist of polioviruses, coxsackieviruses and ECHO viruses. Koplik's spots do not appear in these infections although macular rashes may occasionally be observed.

• Question 383:

  A young child developed staphylococcal scalded syndrome. Which one of the following toxins is most likely responsible for this syndrome?

  A. alpha toxin

  B. erythrogenic toxin

  C. exfoliative toxin

  D. staphylococcal enterotoxins A through F

  E. toxic shock syndrome toxin (TSST)

  Correct Answer: C

  Section: Microbiology/Immunology Exfoliation toxin is produced by certain strains of S. aureus belonging to phage group II. This exotoxin divides the epidermis between the stratum granulosum and the stratum spinosum. The end result of exfoliation action is the exfoliation of the skin, known as the staphylococcal scalded skin syndrome. The TSST (choice E) causes multisystem involvement with shock which can be lethal. TSST is a super antigen that releases large concentrations of IL-1, IL-2, and TNF. Erythrogenic toxin (choice B) is produced by S. pyogenes and is associated with scarlet fever. Alpha toxin (choice A) is produced by S. aureus and causes necrosis and hemolysis of blood cells. Staphylococcal enterotoxins A through F (choice D) are responsible for food poisoning.

• Question 384:

  Agrandmother undergoing chemotherapy for cancer in a hospital was exposed to a grandchild with chickenpox. In order to prevent a clinical complication of varicella or disseminated zoster in the grandmother, which of the materials listed below should be used as an appropriate medical intervention?

  A. acyclovir

  B. indinavir

  C. killed varicella-zoster virus (VZV) vaccine

  D. live attenuated VZV vaccine

  E. subunit VZV vaccine

  Correct Answer: A

  Section: Microbiology/Immunology Prevention of varicella and disseminated zoster rests on the administration of acyclovir. Killed varicellazoster virus (VZV) vaccine has not been shown to be effective. Furthermore, if the immunological system has been compromised the immune responses will be very weak and of questionable value (choice C). Live attenuated VZV vaccines cannot be given to immunocompromised individuals because vaccination can be lethal (choice D). Subunit VZV vaccines are not available, and will be of little value to persons with suppressed immunological responses (choice E). Indinavir is a protease inhibitor used for the treatment of AIDS, not for varicella or zoster (choice B).

• Question 385:

  Ayoung street person was treated with the drug the structure of which is shown in below figure. Which of the following correctly identifies the drug?

  

  A. acyclovir, which inhibits the herpesvirus- encoded DNA polymerase

  B. azidothymidine (AZT), which inhibits the AIDS virus reverse transcriptase

  C. dideoxyinosine, which inhibits poliovirus replication

  D. enviroxine, which inhibits rhinoviruses

  E. idoxuridine, which inhibits herpesvirus thymidine kinase

  Correct Answer: B

  Section: Microbiology/Immunology The antiviral drug currently used against the immunodeficiency virus is AZT, or 3-azido-3- deoxythymidine (azidothymidine). Its structure is shown in below figure. The structures of related compounds idoxuridine (choice E) and dideoxyinosine (choice C) are shown below in below figure. Acyclovir (choice A) and enviroxine,

  

  (choice D) differ chemically from the structure depicted in below figure

• Question 386:

  A Boy Scout troop camped for 1week in the woods in the New England area. After arriving home, two experienced an insect bite lesion that was characterized by a clear center surrounded by a flat reddened area that expanded. They also had fever, chills, fatigue, and headache. Which disease listed below was diagnosed?

  A. epidemic typhus

  B. lyme disease

  C. Q fever

  D. rickettsial pox

  E. trench feve

  Correct Answer: B

  Section: Microbiology/Immunology This patient's symptoms point to a diagnosis of Lyme disease, caused by B. burgdorferi, and transmitted to humans by ticks that harbor this organism. The initial symptoms of Lyme disease are fever, chills, fatigue, and headache, but the pathognomonic feature is a spreading, circular rash with a clear center. The rash begins 318 days after the tick bite and is called erythema chronicum migrans. Q fever (choice C), epidemic typhus (choice A), rickettsial pox (choice D), and trench fever (choice E) do not produce erythema chronicum migrans.

• Question 387:

  A 24-year-old construction worker who had four injections of the DPT (diphtheria, pertussis, and tetanus) vaccine in his first year of life and boosters at ages 5 and 19 received a deep laceration while excavating a building's foundation. Which of the following is the preferred treatment?

  A. equine tetanus immune globulin, because it will passively immunize him

  B. human tetanus immune globulin, because it will stimulate his anamnestic response

  C. penicillin, because it will kill the organism

  D. streptomycin, because it will kill the organism

  E. tetanus toxoid, because it will stimulate his anamnestic response

  Correct Answer: E

  Section: Microbiology/Immunology Since there are memory lymphocytes primed by a previous tetanus toxoid injection, booster immunization with tetanus toxoid will lead to rapid production of adequate levels of protective antibody. This is the routine procedure followed by physicians for trauma patients, who have been accinated against tetanus and received booster immunization within the last 57 years. The antibody titer to tetanus toxoid remains at protective levels for 510 years. Aminoglycosides such as streptomycin (choice D) or penicillin (choice C) will not be effective against the spores or vegetative cells of C. tetani. Human tetanus immune globulin will only provide antibodies to tetanus exotoxin for a short time and cannot induce an anamnestic response because an injection of tetanus exotoxin or toxoid is required (choice B). Individuals who have been vaccinated against tetanus require injections of the tetanus toxoid to induce rapid production of adequate levels of protective antibody. Treatment with equine tetanus immunoglobulin, which will only provide antitetanus antibody for a short time, is not the preferred method of treatment for this patient (choice A).

• Question 388:

  HIV isolated from the same patient over time may differ antigenically. HIV also differs from the viruses found in the general class of RNA tumor viruses. Which of the following best explains how HIV differs from most other RNA tumor viruses?

  A. HIV contains the gag gene

  B. HIV contains the pol gene

  C. HIV contains two copies of singlestranded RNA in its virion

  D. HIV does not require T4 receptor protein for adsorption to host cells

  E. HIV lyses the host cells

  Correct Answer: E

  Section: Microbiology/Immunology An important difference between the AIDS (HIV) virus and the RNA tumor viruses is that HIV lyses the host cells, while RNA tumor viruses transform the cells they invade, butthey lack cytolytic activity. The tropism of the HIV for T4 lymphocytes depends on the presence of the T4 protein on the surface of the lymphocytes. This protein serves as the receptor for the adsorption of the HIV to T4 lymphocytes. The HIV is a member of the retroviruses (choice D). The genomic RNA molecule of HIV contains the gag, pol, and env genes. Thus, the HIV does not differ from RNA tumor retroviruses (choices A and B). HIV contains one copy, not two, of single-stranded RNA in its virion (choice C).

• Question 389:

  Ahemagglutination (HA) assay was performed with influenza virus. Afixed number of chicken red blood cells were mixed with increasing dilutions of the influenza virus. The results of the assay are shown in below figure. Which of the following represents the HA titer of the virus?

  

  A. 20

  B. 40

  C. 80

  D. 160

  E. 320

  Correct Answer: C

  Section: Microbiology/Immunology The ability of certain viruses, such as influenza, mumps, and parainfluenza viruses, to agglutinate red blood cells is used to diagnose these viruses. In general, chicken or human type O red blood cells are employed for the identification of influenza and other viruses. Red blood cells have receptors for the surface component of the influenza virus called hemagglutinin. This hemagglutinin is a glycoprotein. In an HA assay, a fixed number of red blood cells is mixed with increasing dilutions of the influenza virus. Following incubation at 4°C for 2 hours, the tubes containing the red blood cells and the virus are examined for HA. Cells agglutinated by the virus form a lattice that covers the entire bottom of the test tube (virus dilutions 1:20; 1:40; 1:80). The HA titer of the virus is the highest dilution of virus that forms a lattice. In this case, the HA titer is 1:80. Unagglutinated cells form a dark bottom (virus dilutions 1:160; 1:320) (choices D and E).

• Question 390:

  Acollege freshman exhibits symptoms of fever, malaise, sore throat, and fatigue. She also has enlarged lymph nodes and spleen and large, atypical T-lymphocytes. Which of the following tests can be used to confirm which virus is the etiologic agent?

  A. antibody that reacts with Epstein-Barr virus (EBV)-associated nuclear antigen

  B. antibody to hemagglutinin

  C. antibody to neuraminidase (NA)

  D. heterophile antibody that reacts with antigens on sheep erythrocytes

  E. nucleic acid hybridization assays for the presence of Epstein-Barr viral nucleic acid

  Correct Answer: E

  Section: Microbiology/Immunology Infectious mononucleosis is caused by EBV, which is a member of the herpesviruses. Nucleic acid hybridization assays for EBV DNAare the most sensitive means of diagnosing infectious mononucleosis. Hemagglutinins (choice B) and NAs (choice C) are associated with orthomyxoviruses and paramyxoviruses. The majority of infectious mononucleosis patients develop what is known as an heterophile antibody, antibodies that cross-react with unrelated antigens, such as those found on sheep and horse erythrocytes. The heterophile antibody test (choice D) is used for the diagnosis of infectious mononucleosis, but since it is not a very specific test, it is not as good as the nucleic acid hybridization assays for the presence of Epstein-Barr viral nucleic acid. Important antigens that also may be used, but which are less sensitive for diagnostic purposes, include the viral capsid protein (VCA), the early proteins (EA), and the EBV-associated nuclear antigen (EBNA). Infectious mononucleosis patients develop antibody titers exceeding 1:320 and 1:20 against VCA and EA, respectively, during the acute phase of infectious mononucleosis. Antibodies to EBNA develop 12 months after acute infection (choice A).