A patient presents with a brain abscess. The dominant organism is an anaerobe normally found as part
of the oral flora. Which of the following best fits that description?
Only Actinomyces is anaerobic; the rest are aerobic. (And, of those mentioned, only Actinomyces is part of the normal flora. Nocardia can, however, cause brain abscesses as well, but it is acquired from the environment.)
A 23-year-old male who has recently started working on a sheep farm in Nova Scotia develops pneumonia
shortly after helping with lambing. His cough produces little sputum, and a saline-induced sputum sample
shows no predominant organism either with Gram stain or with acid-fast stain. It is established that he
acquired the pneumonia from parturition products from the sheep. Which agent is most likely to be the
cause of his pneumonia?
D. Coxiella burnetii
Coxiella burnetii is a rickettsia-like organism that can be spread via amniotic fluid, aerosols, or dust
particles. It with stands drying and thus can be transmitted at least 10 miles by the wind.
A 3 year old presents with difficulty breathing and will not lie down to be examined. You suspect acute
bacterial epiglottitis and examine the child’s epiglottis, which is highly inflamed. Which vaccine are you
most likely to find that the child is missing?
E. Haemophilus influenzae
Epiglottitis is a medical emergency requiring hospitalization. It can be fatal in 24 hours. Pediatric
cases were almost always caused by H. influenzae type b and have been dramatically reduced by
the conjugate vaccine.
A 22-year-old cystic fibrosis patient presents with fever and increasing dyspnea. A Gram-negative
organism is found in unusually high numbers in the mucus. Which virulence factor is most important
in colonization and maintenance of the organism in the lungs?
C. Polysaccharide slime
Staphylococcus aureus and Pseudomonas aeruginosa are two primary pulmonary colonizers that
cause pneumonia in patients with cystic fibrosis. (Staph is usually only in young CF patients.) Of the two,
Pseudomonas is Gram negative. Its slime material (alginate) produces the resistance to phagocytic
killing and poor penetration of antibiotics to the site, which, in conjunction with the antibiotic resistance of
Pseudomonas , make these serious infections.
Exotoxin A most closely resembles the action of which other microbial toxin?
C. Diphtheria toxin
Both Pseudomonas exotoxin A and diphtheria toxin inhibit protein synthesis through the inhibition of
elongation factor (EF-2). In correct choices include:Shigatoxin, which is a cytotoxin, enterotoxin,
and neurotoxin. Vibrio cholera enterotoxin and E.coli labile toxin (LT) both result in increased cyclic
adenosine monophosphate (cAMP).
A 36-year-old immigrant who lived in a crowded resettlement camp before coming to the United States
now has a cough that has been bothering him for several weeks. He has also lost 10 pounds. Which of
the following factors is known to be most important in triggering the granulomatous reaction to wall off
and contain the infection?
A. Cord factor
Cord factor helps trigger the Th1 response, which helps contain the infection.
Patient develops diarrhea 5 days after starting antibiotic treatment for a serious staphylococcal infection.
What is the most likely causative agent?
B. Clostridium difficile
Clostridium difficile has been shown to be the major causative agent of pseudo-membranous colitis,
which causes diarrhea most commonly starting after 3 to 4 days of antibiotic administration.
A patient develops mild gastroenteritis a few days after having a variety of sushi at a party. There is
no blood or pus in the stool. Which causative agent is most likely to have caused this illness?
B. Vibrio parahaemolyticus
Vibrio cholerae causes classic cholera, which is not generally mild or self-limited;
Vibrio parahaemolyticus , in contrast, causes a relatively mild gastroenteritis. It is also found in raw fish.
Sal.typhi is the causative agent of typhoid. Shigellae infections are always invasive and generally will have
a little pus in the stool.
Yersinia pestis may be transferred by
D. Respiratory droplets
Most transmission in the United States is from infected flea bite (a choice not given in the question).
The other route of transmission is through respiratory droplets from patients who have developed
pneumonic emboli and pneumonia.
A patient had surgery 2 months ago to put in a pace maker. He felt fine for 1 month, but over the past
month, he has been feeling worse. He is running a low-grade fever, tires easily, and has worsening heart
murmurs. Which of the following staphylococcal organisms causes sub-acute bacterial endocarditis
that occurs 2 months or more after heart surgery?
B. Staphylococcus epidermidis
Staphylococcus epidermidis is ubiquitous as part of the normal flora. Organisms are introduced into
the host during invasive procedures. Staph.aureus is more likely to be acute, with high fever and damage
developing more quickly
A previously healthy 6 month old now looks limp. He cannot hold his eyes open, pupils do not react,
and he cannot hold his head up. What is the proper treatment?
D. Monitored supportive care with human botulinum immunoglobulin
Clostridium botulinum found in household dust or honey was ingested by the baby and the spores
germinated in her GI tract because her normal flora was not sufficient to suppress the germination.
It is the vegetative cells that produce the botulinum toxin. Antibiotics disrupt normal flora, prolonging
the disease, but administration of human antitoxin can dramatically reduce the length of the hospital stay.
A 78-year-old man develops a high fever, cough producing a blood-tinged sputum, and difficulty breathing.
Sputum shows organism consistent with Streptococcus pneumoniae. What is the most important virulence
D. Polysaccharide capsule
The Gram-positive organism Streptococcus pneumoniae contains no endotoxin. It is not phagocytosed
in the immunologically naive, eliminating choice B. It is the capsule that is considered the most important
Which of the following organisms grows in 40% bile?
A. Enterococcus faecalis
Enterococci can be differentiated by their reactivity with group D antiserum, bacitracin resistance, and
growth in 40% bile or pH 9.6.
A patient recently returned from Africa has been febrile for several days and now has abdominal pain.
His blood cultures grow out Salmonella typhi. What was the most likely source of his infection?
D. A food preparer with bad personal hygiene
Sal.typhi Has only human hosts
A preterm (33-week) infant girl is born at home to a 16-year-old mom after 22 hours of labor after the
rupture of the membranes. A friend helped the mother deliver the baby. The now 4 day old infant now
shows signs of sepsis. What is the best description for the most likely causative agent? All organisms
in the answer choices are Gram-positive, catalase-negative cocci found in pairs or short chains.
C. Beta-hemolytic cocci in chains and carrying Lancefield’s Group B antigen
If the mother is young and has had multiple sexual partners, she is more likely to be colonized with
Group B streptococci. If the labor is prolonged after rupture of the mem-branes, the baby is more
likely to be infected. And, since she delivered before her due date and had her baby at home, she was
not screened for Group B streptococci and did not receive intra-partum antibiotic stop revent infection
of the baby. The other descriptions belong to:
(B) Strep. pneumoniae ,and
A patient has a gastric ulcer not induced by non-steroidal anti-inflammatory agents. Which characteristic
appears to play a central role in the ability of the organism to survive transit of the lumen to colonize the
B. Urease production
A major survival and virulence factor of Helicobacter pylori is urease, which neutralizes stomach acid
to allow the organism to survive to reach the tissue.
A54-year-old man develops a pyogenic infection along the suture line after knee surgery. The laboratory
gives a preliminary report of a beta-hemolytic, catalase-positive, coagulase-positive, Gram-positive coccus.
The most likely causative agent is
B. Staphylococcus aureus
Of the answer choices, only streptococci and staphylococci are Gram positive. The streptococci are
catalase negative and staphylococci are catalase positive. Of the two staphylococci, Staphylococcus
aureus is the beta-hemolytic, coagulase-positive organism.