USMLE USMLE Certifications USMLE-STEP-1 Questions & Answers

• Question 461:

  Severe septic infections caused by bacteriapossessing capsules often occur following splenectomy. Which of the following statements most accurately explains why this medical complication is able to occur?

  A. The spleen is a source of IgM and IgG antibodies needed to opsonize encapsulated bacteria.

  B. The spleen is the main source of stem cells.

  C. The spleen is uniquely equipped to process capsular polysaccharides.

  D. The spleen rapidly inactivates many antimicrobial agents.

  E. The spleen readily metabolizes therapeutic doses of antibiotics.

  Correct Answer: A

  Section: Microbiology/Immunology The spleen is important for the production of IgM and IgG antibodies needed to opsonize encapsulated bacteria. Involvement of the spleen in metabolizing (choice D), inactivating of antimicrobial agents (choice E), or processing of capsular polysaccharides (choice C) is not considered important. The main source of stem cells is the bone marrow not the spleen (choice B).

• Question 462:

  A 38-year-old sexually-active woman developed symptoms of lower abdominal pain with adjacent tenderness. No gram-negative cocci were observed or isolated. The obligate intracellular organism causing the infection is unable to perform which of the following metabolic functions?

  A. form ATP

  B. form the intracellular, metabolically active, reticulate body

  C. produce polypeptides

  D. reproduce by binary fission

  E. synthesize DNA gyrase

  Correct Answer: A

  Section: Microbiology/Immunology Chlamydae are obligate intracellular parasites because they cannot synthesize ATP. They are classified as bacteria because they divide by binary fission (choice D), form polypeptides (choice C), produce RNA, DNA, and DNA gyrase (choice E). Finally, they synthesize two unique morphological forms. That is, the extracellular infective, metabolically inert elementary body, and the intracellular, metabolically active reticulate body (choice B).

• Question 463:

  A 25-year-old man with acquired immune deficiency disease (AIDS) has malabsorption, chronic abdominal pain, low-grade fever, and nonbloody diarrhea. In fecal smears, large numbers of oocysts containing four sporozoites were observed. Which of the following organisms is most likely the etiological agent?

  A. Cryptosporidium parvum

  B. Entamoeba histolytica

  C. Giardia lamblia

  D. Microsporidia species

  E. Taenia solium

  Correct Answer: A

  Section: Microbiology/Immunology All five organisms are protozoa. There are two intestinal protozoa specifically associated with AIDS that can cause transient diarrhea in immunocompetent individuals, but can cause debilitating, and potentially life- threatening, chronic diarrhea in AIDS patients. These organisms are C. parvum with no effective drug therapy, and Isospora belli treated with folate antagonists, such as trimethoprim- sulfamethoxazole. Microsporidia may cause severe, persistent, watery diarrhea in AIDS patients, but they produce spores rather than oocysts (choice D). T. solium is the pork tapeworm which occasionally may cause diarrhea (choice E). E. histolytica and G. lamblia may cause diarrhea, but they are not closely associated with AIDS (choices B and C).

• Question 464:

  Which of the following statements best describes interferon's suspected mode of action in producing resistance to viral infection?

  A. It alters the permeability of the cell membrane so that viruses cannot enter the cell.

  B. It stimulates a cell-mediated immunity.

  C. It stimulates humoral immunity.

  D. Its action is related to the synthesis of a protein that inhibits mRNA function.

  E. Its direct antiviral action is related to the suppression of messenger RNA formation.

  Correct Answer: D

  Section: Microbiology/Immunology Interferons are produced by most nucleated cells in the body, and inhibit the translation of viral proteins. There are three major types of interferons, that is, alpha, beta, and gamma. Interferons are active against many viruses and are not specific. Virus infection in one cell induces interferon production which moves to an adjacent, uninfected cell and causes an antiviral protein (AVP) to be produced by the second cell. This AVP inhibits viral mRNA activity and thereby inhibits viral protein production. Interferons do not destroy viruses, but probably result in fewer virions that need to be eliminated by the host's immune system. Interferons do not affect host cell membranes (choice A) or directly suppress mRNA function (choice E). Interferons do not directly stimulate either cellular (choice B) or humoral (choice C) immunity to enhance an antiviral effect.

• Question 465:

  In February, three patients from a nursing home in East Dallas develop an abrupt onset of a febrile upper respiratory tract illness. They have received no immunizations in the last year. Culture of the nasopharynx of one patient shows influenza A virus. To prevent further cases within the next two weeks, which of the following should be administered?

  A. acyclovir

  B. amantadine

  C. hyperimmune serum globulin

  D. influenza A vaccine

  E. ribavirin

  Correct Answer: B

  Section: Microbiology/Immunology Influenza viruses are helical viruses with a segmented, negative-sense RNA genome. A, B, and C types are determined by the protein coat of the virus. The infections are characterized by short (12 days) incubation periods and are efficiently transmitted in respiratory secretions. While most RNA viruses grow only in the infected cell cytoplasm, influenza viruses have growth steps in both the cytoplasm and nucleus of the host cell. The patient's humeral immune system is most important in controlling virus growth. The important antibodies are made against the viral hemeagglutinin (early viral attachment) and the NA (release of mature viruses) enzymes. Amantadine (choice B) and rimantadine block viral uncoating and must be given prophylactically. Oseltamivir (Tamiflu) inhibits NA activity and must also be given before viral infection for best results. Acyclovir (choice A) is an antiherpetic drug. Passive immunity (choice C) does not play a significant role in influenza control. Ribavirin (choice E) is used to treat RSV, a paramyxovirus. Active immunity (choice D) is the most effective mechanism of controlling influenza on a population basis and probably would not be as effective as prophylactic antivirals in the nursing home arena.

• Question 466:

  Physical examination reveals severe laryngeal edema resulting in a marked hoarseness when the 17year- old girl attempts to speak. Also, her lips and tongue show significant swelling. Which of the following chemicals most likely caused this edematous swelling?

  A. corticosteroids

  B. cromolyn sodium

  C. epinephrine

  D. histamine

  E. 5-lipoxygenase

  Correct Answer: D

  Section: Microbiology/Immunology Histamine is among the class of preformed (primary) mediators released from mast cells upon degranulation and is classified as a biogenic amine. It is a potent pharmacologically active substance that can dilate small blood vessels and increase vascular permeability, allowing fluid to leak into tissues. These vascular changes can give rise to swelling and redness. Histamine can stimulate production of mucus from the epithelium, thus contribute to congestion of airways. It can cause smooth muscles around the bronchial airways and the intestine to contract. Prostaglandins and leukotrienes like histamine can also cause constriction of the bronchial tubes and vascular dilation. Corticosteroids are anti-inflammatory drugs that are used for the treatment of asthma, autoimmune diseases, or graft rejection (choice A). Epinephrine and cromolyn sodium counteract the action of histamine or other mediators of type I hypersensitivity reaction and are used for the treatment of anaphylactic reactions (choices B and C). 5- lipoxygenase retards production of several leukotrienes (choice E).

• Question 467:

  Following the consumption of fish at a local restaurant, a 17-year-old girl suddenly experiences severe dyspnea. Upon arrival at the local emergency room she is found to be severely hypotensive and in respiratory distress. Urticarial wheals are also noted all over her body. As the resident physician you quickly realize this to be a hypersensitivity reaction. To which of the following immunoglobulins does the allergen bind rapidly during the activation stage of mast cells?

  A. IgA

  B. IgD

  C. IgE

  D. IgG

  E. IgM

  Correct Answer: C

  Section: Microbiology/Immunology Within seconds after the allergen encounters human tissue, it binds to IgE antibodies which are attached to mast cells. When it engages at least two IgE molecules it forms a bridge between them. This cross-linking, in turn, draws the attached IgE receptors close to one another. Such aggregation of receptors activates mast cells to release histamine and other vasoactive substances that generate the allergic reaction. IgA, IgG, IgM, or IgD are not the germane antibodies to which allergens bind (choices A, B, D, and E).

• Question 468:

  An emaciated prisoner in a New York prison began coughing up sputum streaked with blood. Examination of the sputum revealed the presence of acid-fast bacilli. Which of the following would be least helpful in establishing a diagnosis of tuberculosis?

  A. agglutination test for antibodies

  B. niacin production test

  C. nitrate reduction test

  D. rough, non-pigmented colonies

  E. slow growth on Lowenstein-Jensen medium

  Correct Answer: A

  Section: Microbiology/Immunology Molecular amplification techniques provide fast diagnostic results when they are compared to traditional cultural procedures for M. Tuberculosis, which often has to be incubated for several weeks. The rodshaped, aerobic bacteria do not stain readily, but once stained they resist decoloration by acid or alcohol (hence, acid-fast bacilli). Cellular immunity is most important in how a host controls the growth and spread of the tubercle bacteria. There is evidence that mononuclear phagocytes ingest the organisms and even destroy them. The tuberculin skin test is based on cellular immune mechanisms, with the PPD causing a positive skin reaction in a person who has encountered the mycobacterium in the past. As a result, humoral immunity reactions are of little use diagnostically (choice A). Other mycobacteria are capable of accumulating niacin (choice B) and reducing nitrate (choice C). The development of rough, non-pigmented colonies on media (choice D) and slow growth on L-J medium (choice E) are also not specific for M. tuberculosis since slow growth (weeks) and colony morphology (rough, no pigment) are characteristic of other species of Mycobacterium.

• Question 469:

  A 10-year-old boy has a bacterial pneumonia. The etiologic agent had M type-1 protein and grew on blood agar. The patient's immune system appeared incapable of controlling the infection. Adecrease in which of the following is most directly related to the immunological failure?

  A. antibody activity

  B. cytotoxic T-cell function

  C. IL-4 and IL-10 levels

  D. mast cell degranulation

  E. platelet aggregation

  Correct Answer: A

  Section: Microbiology/Immunology A deficiency in immunoglobulin production or activity is directly related to susceptibility to common extracellular microbes such as streptococci. Cytokines such as the interleukins, IL-4 and IL-10 (choice C), the function of T cells (choice B) or mast cells (choice D), and nonantibody factors (choice E) are not directly involved in increased susceptibility to bacterial infections.

• Question 470:

  A student has a genetic defect and cannot produce the J chain which is important in the structure of some immunoglobul in molecules. Which of the following will most likely be observed in this individual?

  A. a decrease in mature B cells

  B. a decrease in mature T lymphocytes

  C. a decrease in serum IgM

  D. an increase in IgA in the intestine

  E. an increase in serum IgM and decrease in IgE

  Correct Answer: C

  Section: Microbiology/Immunology Deficiency in production of J chains for immunoglobulins could result in a decrease in serum IgA and IgM levels. These antibodies are dimers and pentamers, respectively, of the basic immunoglobulin molecule, and require a J chain to join the immunoglobulin chains. There is no evidence which indicates that a deficiency in the production of J chains of IgA or IgM will cause a decrease in mature B cells, or T lymphocytes, an increase in serum IgM and decrease in IgE, or a decrease in IgA (choices A, B, D, and E).