USMLE USMLE Certifications USMLE-STEP-1 Questions & Answers

• Question 441:

  Darrin is an intern working in the emergency room at the local county hospital. Mr Arnold, a 48-year- old homeless man, comes in complaining of "feeling real bad." Mr Arnold's mentation is altered, and is unable to verbalize his past medical history. Darrin is having difficulty examining the patient because his body emits a tremendous odor. While examining him, Darrin discovers that his right leg is red, swollen, sore, and covered with ulcers. The nurse in the emergency room covers the leg with dressings and a plastic bag to contain the smell. By morning, Mr Arnold's anaerobic blood cultures become positive with grampositive rods. Darrin observes that the bag is full of air, and when he removes it the smell is overwhelming. What should Darrin do?

  A. Arrange for Mr Arnold to receive hyperbaric oxygen therapy.

  B. Consult with a surgeon, as Mr Arnold will need debridement and IV antibiotics.

  C. Continue the broad spectrum IV antibiotics he has prescribed for Mr Arnold.

  D. Put Mr Arnold in respiratory isolation and confirm his diagnosis with a spinal tap.

  Correct Answer: B

  Section: Microbiology/Immunology Darren's patient is most likely suffering from an infection that is caused by toxin-producing clostridia. C. perfringens and related clostridia can produce invasive infection (myonecrosis and gas gangrene) if they are introduced into damaged tissue. These organisms produce a large variety of toxins that may be lethal, necrotizing, and/or hemolytic. Clostridia are anerobic bacteria and can only grow well in conditions of low oxygen contact, such as necrotic tissue. In such an infection, within a few days there is foul-smelling discharge, fever, hemolysis, toxemia, shock, and death. Treatment includes immediate surgery and antibiotic administration. Hence, choice B would be the next appropriate medical intervention. Hyperbaric oxygen (choice A) has no proved effect in this situation. Antibiotics only (choice C) would provide only partial resolution and not in a timely manner. Choice D has nothing to do with the medical management of Darren's patient.

• Question 442:

  A 60-year-old male with a chronic cough, bloody sputum, and marked weight loss is diagnosed as having tuberculosis. A "serpentine-like" colonial morphology is noted on Lowenstein- Jensen agar. This latter finding is due to which of the following factors?

  A. coagulase

  B. endotoxin

  C. large "slimy" capsule

  D. trehalose-6,6'-dimycolate

  E. Wax D

  Correct Answer: D

  Section: Microbiology/Immunology Mycobacteria are rich in lipids. These include mycolic acids, waxes, and phosphatides. In the cell, the lipids are largely bound to proteins and polysaccharides and are, to some extent, responsible for acid-fastness. Virulent strains of tubercle bacilli form microscopic "serpentine cords" in which acid-fast bacilli are arranged in parallel chains. Cord formation is correlated with virulence and a "cord factor" (trehalose-6, 6-dimycolate choice D) has been extracted from virulent bacilli. It inhibits leukocyte migration, causes chronic granulomas, and may serve as an immulogical adjuvant. Virulent mycobacteria do not have endotoxin (choice B) as do the enteric bacteria which may cause inflammation. Nor do mycobacteria produce coagulase (choice A). Mycobacteria colonies are usually rough and dry, not "slimy" from a capsule (choice C). Wax D (choice D) is present in mycobacteria, but is not associated with cord factor.

• Question 443:

  Aboy picked up a grounded bat and was bitten on the hand, suffering a slight wound. In addition to flushing the wound, cleaning it surgically, and giving antitetanus prophylaxis and antibiotics as indicated, the physician should immediately perform which of the following?

  A. observe the boy very carefully

  B. order a search for the attacking bat for autopsy

  C. report the incident to the state epidemiologist

  D. start rabies vaccine

  E. start rabies vaccine and give antirabies serum

  Correct Answer: E

  Section: Microbiology/Immunology If exposure to rabies virus appears definite, as in this case, treatment with human diploid cell live- derived vaccine and hyperimmune antirabies gamma globulin should be started immediately. Serum antibodies provide an immediate barrier to the growth of virus; meanwhile, antibodies are elicited by the vaccine. If the level of exposure is minimal (e.g., no skin puncture) and the animal probably is not rabid, vaccine is not recommended. Ordering a search for the attacking fox for autopsy to determine if it has rabies is like searching for a needle in a haystack, and thus not the best approach to address a possible rabies infection, which requires immediate actions (choice B). Initiation of rabies vaccine will lead to production of antibodies against the rabies virus, but it will require approximately 2 weeks to develop protective antibodies. By that time, severe damage may have already occurred (choice D). Postexposure immunization and human rabies immunoglobulin is the best approach. Observation of the patient (choice A) or reporting of the incident to the state epidemiologist (choice C) do not address the real needs of an individual running the risk of rabies.

• Question 444:

  Which of the following approaches is likely to prevent future infection of patrons of the sex worker in question 24?

  A. frequent washing of the genital area

  B. immunization with the appropriate toxoid

  C. prophylactic use of 100 units oral penicillin

  D. use of condoms

  E. vaccination with heat-killed germs

  Correct Answer: D

  Section: Microbiology/Immunology Prevention of gonorrhea rests on the use of safe, properly worn condoms, and immediate treatment of symptomatic patients as well as their sexual partners. Ceftriaxone is the best treatment for uncomplicated cases of gonococcal infections. Since 1976, penicillinase-producing strains of N. gonorrhoeae have been isolated that show a high level of resistance to penicillin. Thus, use of 100 units of penicillin will be of little if any value for the prevention of gonorrhea (choice C). There are no vaccines composed of either killed germs (choice E) or toxoids (choice B) available for the prevention of gonorrhea. Frequent washing of the genital areas is not likely to prevent gonorrhea (choice A).

• Question 445:

  A 20-year-old prostitute presents at a local clinic with fever and abdominal tenderness at day 2 of her menstrual cycle. Gram-negative diplococci are seen in a Gram stain of vaginal secretions with many of the bacteria inside PMNs. Which of the following is likely the most important virulence factor necessary for this organism to initiate infection?

  A. capsule

  B. endotoxin

  C. hyaluronidase

  D. pili

  E. protease

  Correct Answer: D

  Section: Microbiology/Immunology The most important presumptive diagnostic test for gonorrhea is the demonstration of gram-negative, kidney-shaped diplococcic inside PMNs obtained from thick, creamy urethral exudates. Outside the human host, N. gonorrhoeae is fragile and usually do not survive well and are rapidly killed by drying, heat, and many disinfectants. Inside the host, however, the cocci possess virulence factors that enhance the organism's ability to establish mucus membrane infections. Pili (choice D) are hair-like appendages that extend out several micrometers. They enhance attachment to host cells and resistance to phagocytosis and are the most significant virulence factor. Polysaccharide capsules (choice A) do not appear to be as significant for gonococci infection as they do for N. meningitides infections. N. gonorrhoeae endotoxin (choice B) or LPS is a significant factor for toxicity once infection is established. A protease (choice E) can split IgA molecules, inactivating those antibodies, as do the meningococci. Hyaluronidase (choice D) can be an important virulence factor for grampositive cocci.

• Question 446:

  below figure depicts an electrophoretic pattern of serum proteins. This electrophoretic pattern is likely to be obtained from a 6-month-old infant suffering from recurrent pyogenic infections. He has which of the following diseases?

  

  A. Addison disease

  B. Bruton disease

  C. Graves disease

  D. Goodpasture syndrome

  E. Myasthenia gravis

  Correct Answer: B

  Section: Microbiology/Immunology Bruton disease is an X-linked congenital agammaglobulinemia. Thus, electrophoresis of the serum of infants, after the passively transferred maternal antibodies have been eliminated, reveals the absence or very low levels of gamma globulin. In contrast to Bruton disease all the other diseases listed will reveal a gamma globulin peak. Goodpasture syndrome (choice C) and Addison disease (choice A) are caused by autoantibody production to basement membrane of kidney and adrenal cortex, respectively. Myasthenia gravis (choice E) involves autoantibody production to acetylcholine receptor of the neuromuscular junctions. Graves disease (choice D) is caused by autoantibody production to thyroid- stimulating hormone receptors.

• Question 447:

  An outbreak of severe, atypical pneumonia occurred in a Southeast Asian community. Symptoms included high fever, chills, headache, and a cough or breathing difficulty. Avirus with the structure shown in below figure was implicated as the cause of the infections. Which of the viruses listed below caused this outbreak?

  

  A. arenavirus

  B. filovirus

  C. poxvirus

  D. rabies virus

  E. SARS virus

  Correct Answer: E

  Section: Microbiology/Immunology It has recently been shown that severe acute respiratory syndrome or SARS is caused by a corona-associated virus. This virus is a positive, single-stranded RNA, enveloped virion which has petal-like projections called corona, and is shown in below figure. The structure of rabies virus is bullet-shaped (choice D). Poxvirus is a very large, brick-shaped, very complex virus (choice C). Filovirus, such as the Ebola, or Marburg virus is very long and filamentous (choice B). Arenaviruses, like the lymphocytic choriomeningitis, or Lassa fever virus are circular, and have granules (arena) of nonfunctional ribosomes on their surfaces (choice A).

• Question 448:

  Which of the following statements concerning total body energy storage is correct?

  A. Most of the body's energy store is held as carbohydrate.

  B. Most of the body's energy store is held as lipid.

  C. Most of the body's energy store is held as plasma glucose.

  D. Most of the body's energy store is held as protein.

  E. Total body's energy storage approximately equals resting metabolic rate.

  Correct Answer: B

  Section: Physiology Lipid is the most concentrated form of energy storage, holding 9.4 kcal/g. For a typical 70 kg human over 130,000 kcal is typically stored as fat. Storage of energy as available protein (choice D) is about 20,000 kcal, while storage as carbohydrate (choice A) is about 3,000 kcal. Though blood glucose (choice C) is critically important, in particular as a source of energy for the brain, blood glucose does not provide a significant fraction of total body energy storage. While total body energy stores typically exceed 150,000 kcal, resting metabolic rate (depending on age and body mass) is about 2100 kcal/day (choice E).

• Question 449:

  During a brain surgery, during which the patient remained conscious, a part of the DC/ML (dorsal column, medial lemniscal) system was exposed and stimulated. Which of the following sensations might the patient most likely experience?

  A. a feeling as if his fingers are warming up

  B. a feeling as if something were touching his index finger

  C. a tingling sensation

  D. mild pain but he is unable to explain exactly where it hurts

  Correct Answer: B

  Section: Physiology The dorsal column-medial lemniscus (DC/ML) system is the sensory pathway that transmits conscious proprioceptive information with fine gradations such as a fine touch on the index finger to the cerebral cortex. The brain interprets any signals coming along the "labeled lines," the nerves that carry messages from the sensory organ to the cortex, as being information about a particular sense. Hence, activation of certain neurons in the somatic sensory system can cause a sensation as if it originated from mechanical pressure on the finger (choice E). Thermal sensations (choice A), tingling (choice C), and crude undetermined pain (choice D) are sensory modalities that are processed in the spinothalamic or anterolateral system.

• Question 450:

  A 22-year-old Hispanic man worked for a company that processed animal products and provided no medical coverage benefits. One week after working with wool imported from a Caribbean island, he developed a small lesion on his arm resembling an insect bite. One week later, the lesion was 2.5 cm in diameter with a central, black sloughed-skin area. Two weeks later, he presented at an emergency room with early stages of sepsis. Which of the following microbes is responsible for the infection?

  A. Bacillus anthracis

  B. Bacteroides melanogenicus

  C. Hemophilus ducreyi

  D. M. scrofulaceum

  E. Treponema pallidum

  Correct Answer: A

  Section: Microbiology/Immunology A patient with cutaneous anthrax usually presents with a pruritic macule or papule that enlarges into a round ulcer in several days. Then 12 mm vesicles that discharge clear or sanguinous fluid might appear. Purulent drainage ordinarily does not occur. This is followed by the development of a painless, depressed, blackcolored eschar. If it is not treated early, dissemination of the disease in about 10% of cases results in septicemia and meningitis. H. ducreyi (choice C) produces a disease called chancroid or soft chancre, which is not associated with the formation of black eschars. T. pallidum (choice E) produces hard chancres without black eschars. B. melanogenicus (choice B) causes abscesses. M. scrofulaceum (choice D) is associated with scrofula which is cervical adenitis.