USMLE USMLE Certifications USMLE-STEP-1 Questions & Answers

• Question 331:

  A 62-year-old woman has had atrial fibrillation since experiencing a myocardial infarction (MI) 7 months prior. Two weeks ago she was hospitalized following a car accident in which she suffered a compound fracture of her left femur and several severe contusions. She now returns to the emergency room with right flank pain, hematuria, and left-sided paralysis. These newly developing problems are most likely the result of which of the following?

  A. air embolism from the compound fracture

  B. bone marrow embolus from the fractured femur

  C. fat embolism from the fractured femur

  D. systemic thromboemboli from the left atrium

  E. venous thromboemboli from the deep leg veins

  Correct Answer: D

  Section: Pathology and Path physiology Atrial fibrillation produces turbulence that is conducive to the formation of thrombi which can then embolize throughout the systemic circulation. In this patient the right flank pain and hematuria and left- sided paralysis suggest that thromboemboli traveled to the right kidney and the brain, respectively. Embolism of air (choice A) has variable effects, although small amounts are typically inconsequential. Larger volumes (~ >100 mL) can obstruct arteries and lead to ischemia and necrosis; an air embolism that lodges in the right heart may lead to sudden death. However, air embolism is rare and is not the most likely event in this patient. Bone marrow embolism (choice B) can occur following a broken bone or cardiac resuscitation, but typically will have no clinical consequences. However, fat embolism (choice C) from a broken long bone or traumatized areas of adipose tissue can, when severe, produce clinical manifestations. These follow the trauma by about 13 days and usually include dyspnea, skin rash, and acute neurologic changes. In a few cases this has been fatal. Venous thromboemboli from deep leg veins (choice E) could not get past the lungs (with the unusual exception of paradoxical embolism in a person with a septal defect) so could not reach the kidneys or brain to cause the effects reported for this patient.

• Question 332:

  A 48-year-old man had a long history of alcoholism (including alcoholic hepatitis and hallucinations) and was admitted to the ICU with hypotension and GI bleeding. He was given IV fluid and transfused with packed RBCs. He remained intubated and ventilator dependent for several weeks. He developed a high fever and was treated with broad-spectrum antibiotics. Culture of his tracheal aspirate initially grew S. aureus. After more antibiotic treatment, Gram stain of his aspirate showed PMNs and gram- negative rods. A chest x-ray demonstratedan infiltrate with possible small abscesses. Tracheal aspirate then yielded a heavy growth of a gram-negative, nonfermenting rod that produced a greenish hue in the culture plates. Which of the following is the most likely organism causing this patient's problem?

  A. H. influenza

  B. L. pneumophilia

  C. M. pneumonia

  D. P. aeruginosa

  E. S. pneumoniae

  Correct Answer: D

  Section: Microbiology/Immunology The pseudomonads are gram-negative, motile, aerobic rods that produce water-soluble pigments. They occur widely in soil, water, plants, and animals. P. aeruginosa is frequently present in small numbers in the normal intestinal tract and on the skin of humans. It is also commonly present in moist environments in hospitals. While a saprophyte on normal immune-competent humans, it is a most efficient opportunist in people with deficient host defenses. Choice D is the correct answer. S. pneumoniae (choice E) are grampositive, and Mycoplasma species (choice C) have no cell wall but are so small that they would not be observed in a Gram stain preparation. H. influenzae (choice A) are extremely small gram-negative rods, much smaller than pseudomonads. Legionella (choice B) stain poorly by the Gram stain procedure and are not seen in clinical specimens.

• Question 333:

  A 10-year-old boy living in a wooded area of northern California is taken to the emergency room after developing a typical erythema migrans (EM) rash around what appears to bea tick bite, and complaining of severe headache. Which of the following is the most likely diagnosis?

  A. Lyme disease caused by B. afzelii and transmitted by the hard-bodied tick Ixodes pacifica

  B. Lyme disease caused by B. burgdorferi and transmitted by the hard-bodied tick I. pacifica

  C. Lyme disease caused by B. burgdorferi and transmitted by Anopheles mosquitoes

  D. relapsing fever caused by B. dudonii and transmitted by a soft-body tick of the Ornithodoros species

  E. relapsing fever caused by B. recurrentis and transmitted by a body louse (Pediculus humanus)

  Correct Answer: B

  Section: Microbiology/Immunology Lyme disease is named after the town of Lyme, Connecticut, where clusters of cases were identified in children. Lyme disease is caused by the spirochete B. burgdorferi, which grows most readily in a complex liquid medium. It has been isolated most easily from the characteristic skin lesion called erythema migrans.

  B. burgdorferi is transmitted by a small, hard-bodied tick of the genus Ixodes. The vector in the Northeast and Midwest is I. scapulous and I. pacificus on the West coast of the United States. There is often flu-like illness when the erythema migrans lesion (flat, reddened area with central clearing) is present. Late manifestations of arthralgia and arthritis are seen if early diagnosis and treatment (doxycycline or amoxicillin) is missed. Only B. burgdorferiappears to be etiologic and only hard-bodies Ixoid ticks appear to be vectors.

• Question 334:

  A 60-year-old man arrives at the emergency room with chills, high fever, headache, and muscle pain. He is a slaughterhouse worker. Believing he had a flu, the man had stayed at home for 9 days after the onset of the symptoms. You suspect a diagnosis of leptospirosis. What body fluid should the clinical laboratory check for the presence of L. interrogans?

  A. blood

  B. saliva

  C. serum

  D. spinal

  E. urine

  Correct Answer: E

  Section: Microbiology/Immunology

  Leptospirosis is a zoonosis of worldwide distribution and is caused by spirochetes of the genus Leptospira.

  L. interrogans is the main pathogen. Leptospirae grow best under aerobic conditions at 28 30°C in seru m-containing semisolid media. After 12 weeks, the leptospirae produce a diffuse zone of growth near the top of the tube and later in a ring corresponding to the level of optimal oxygen tension for the organisms. Human infection results usually from ingestion of water or food contaminated with the bacteria. The organisms establish themselves in the liver and kidneys. Kidney involvement in many animal species is chronic and results in the shedding of many leptospirae in the urine (choice E). Other body fluid choices (A, B, C, and D) are not preferred specimens for leptospirae isolation.

• Question 335:

  Mumps virus accounts for 1015% of all cases of aseptic meningitis in the United States. Which of the following is most true of infection with mumps virus?

  A. It is apt to recur periodically in many affected persons.

  B. It is maintained in large canine reservoir.

  C. It is preventable by immunization.

  D. It usually produces severe systemic manifestations.

  E. It will always cause mumps orchitis in postpubertal males.

  Correct Answer: C

  Section: Microbiology/Immunology Mumps is an acute contagious disease characterized by nonsupportive enlargement of one or both salivary glands. While it is estimated that one-third of all infections are subclinical, mumps in children is mild. In adults, complications include meningitis and orchitis. Immunity is permanent after a single infection or vaccination with the single antigenic type live-attenuated vaccine (choice C). As stated, it does not recur in infected individuals (choice A) and is a human pathogen, not canine (choice B). Meningitis and orchitis are the main complications, but severe systemic manifestations (choice D) do not occur. Orchitis (choice E) occur in 2050% of infected men.

• Question 336:

  Asmall gram-negative pleomorphic organism was isolated from the nasal secretion of a toddler who later had paroxysms of coughing and post-tussive vomiting lasting at least 2 weeks. Which of the following culture media should be used to culture the suspected organism?

  A. blood agar

  B. Bordet Gengou agar

  C. MacConkey agar

  D. Thayer Martin agar

  E. Tryptic soy agar

  Correct Answer: B

  Section: Microbiology/Immunology

  B. pertussis is a small gram-negative rod that produces a number of virulence factors that are involved in the pathogenesis of whooping cough (pertussis). The organism has a capsule that inhibits phagocytosis and is susceptible to several antibiotics. The pertussis vaccines (containing 15 antigens) provide excellent active immune protection. B. pertussis can be isolated and identified using Bordet- Gengou (choice B) medium. Blood agar (choice A) is a general enriched medium but is insufficient to isolate B. pertussis. MacConkey agar (choice C) is routinely used to isolate and identify lactose- positive and lactose-negative enteric bacteria. Thayer Martin agar (choice D) is used for the isolation of pathogenic neisseria species. Tryptic Soy agar (choice E) is a general medium that will support many species of normal flora bacteria.

• Question 337:

  A number of children are hospitalized with bloody diarrhea and severe hematological abnormalities. A4year-old girl dies of kidney failure shortly after admittance. Epidemiological investigation establishes that all of the patients developed symptoms soon after consuming hamburgers from the same fast-food restaurant chain. Which of the following organisms is most likely to be responsible for the outbreak?

  A. C. jejuni

  B. non-01 serogroup of V. cholera

  C. O157:H7 serotype of E. coli

  D. S. typhimurium

  E. Shigella dysenteriae

  Correct Answer: C

  Section: Microbiology/Immunology The enterobacteriaceae are a large, heterogeneous group of gram-negative rods that normally inhabit the intestinal tract of humans and animals. The Escherichia genus (usual flora) can incidentally cause disease while Salmonella and Shigella genera are regularly pathogenic for humans. E. coli is the most common cause of urinary tract infection, but there are also several E. coli-associated diarrheal diseases that occur worldwide. Enterohemorrhagic E. coli (EHEC) produces verotoxin and is associated with hemorrhagic colitis and hemolytic uremic syndrome. E. coli serotype O157:H7 (choice C) is the most common strain for these disease presentations. Many outbreaks could be prevented by thoroughly cooking ground beef. EPEC is a strain that causes diarrhea in infants while ETEC is a common cause of traveler's diarrhea. C. jejuni (choice A) can be responsible for a shigella-like illness that is self- limited. Milk or undercooked fowl are frequent vectors of this organism. Non- 01 serogroup of V. cholerae (choice B) only causes a cholera-like illness occasionally. Salmonella (choice D) enterocolitis can be caused by 1400 serotypes. Nausea, headache, vomiting, and profuse diarrhea with low-grade fever are common and resolve in 23 days. The natural habitat of shigellae (choice E) is limited to the intestinal tract of humans and other primates, where they produce bacillary dysentery.

• Question 338:

  Which pair of organisms below can be routinely responsible for food poisoning?

  A. C. botulinum and B. anthracis

  B. C. difficile and C. botulinum

  C. C. perfringens and B. cereus

  D. C. tetani and B. anthracis

  E. C. tetani and B. cereus

  Correct Answer: C

  Section: Microbiology/Immunology

  C. perfringens (choice C) may well cause myonecrosis and gas gangrene in an invasive infection due to the many tissue-destroying toxins the bacteria can make. However, a powerful enterotoxin that induces intense diarrhea in 618 hours can be made by C. perfringens. It is similar to food poisoning caused by B. cereus (a soil organism) and tends to be self-limited. B cereus (choice C) produces an emetic type of food poisoning and a diarrheal form by toxins produced by the bacteria. It has a short incubation (hours) and recovery usually occurs in 24 hours. C. botulinum (choice A) produces a toxin that is ingested and affects the CNS in 24 days. Gastrointestinal anthrax (choice A) is quite rare. C. difficile (choice B) become predominant in the intestinal tract as a result of antibiotic treatment. It produces two toxins that cause pseudomembranous colitis. C. tetani (choice D) causes tetanus, not an enteric disease.

• Question 339:

  A Gram stain smear of spinal fluid from a 2- year-old child reveals short gram-negative rods. Which of the following most likely is the organism?

  A. A. israelii

  B. B. fragilis

  C. E. coli

  D. H. influenza

  E. L. monocytogenes

  Correct Answer: D

  Section: Microbiology/Immunology Haemophilus species are a group of small gram-negative pleomorphic bacteria thatrequire blood agar enriched with X and/or V factors for isolation. H. influenzae (choice D) is an important pathogen found on the mucus membranes of the upper respiratory tract in humans. It is an important cause of meningitis in children, and it would not be unusual to isolate the organism from spinal fluid in such a case. Children aged 6 months5 years are the group most at risk. H. influenzae is also able to cause respiratory tract infections in children and adults. Such infections can be controlled by use of the H. influenzae type b conjugate vaccine as part of the pediatric vaccination series. Actinomyces (choice A) species are gram-positive rods, usually found in the oral cavity. B. fragilis (choice B) is a gram- negative rod usually found in the intestinal tract and may be opportunistic. These are anerobes, and appropriate conditions must be used for isolation. E. coli (choice C) is a larger enteric bacterium, also a very efficient opportunist, which will grow on less complex media. L. monocytogenes (choice E) is a grampositive organism.

• Question 340:

  A 14-year-old boy presents with acne with inflamed follicles and sebaceous glands. Gram stain of a pustule from his face reveals pleomorphic gram-positive bacilli. Which of the following is the organism implicated in this condition?

  A. C. durum

  B. Erysipelothrix rhusiopathiae

  C. L. monocytogenes

  D. S. aureus

  E. S. epidermidis

  Correct Answer: A

  Section: Microbiology/Immunology The non-spore forming gram-positive bacilli are a diverse group of bacteria. While C. diphtherae is the most important pathogen, many corynebacteria and propionibacteria (anerobes) are members of the normal flora of skin and mucus membranes. These bacteria tend to be clubbed or irregularly shaped. C. durum (choice A) is a diphtheroid, a normal flora species that can be opportunistic in the right location, such as hair follicles and sebaceous glands. It may contribute to lesion development or be part of a mixed culture of skin bacteria present in the pustule. E. rhusiopathial (choice B) causes disease in domestic swine. Erysipelas in humans is caused by Group A streptococci. L. monocytogenes (choice C) enters the body through the GI tract after ingestion of contaminated cheese or vegetables. S. aureus (choice D) and

  S. epidermidis (choice E) are welldefined gram-positive cocci.