USMLE : Microbiology Toughest Test! Trivia Quiz

The correct answer is E. The combination of optochin sensitivity and positive Quellung reaction are properties
of a single organism, Streptococcus pneumoniae. The other encapsulated organisms that have
Quellung-positive reactions are Haemophilus influenzae(choice B), Neisseria meningitidis, and Klebsiella
pneumoniae(choice C). However, none of these organisms are optochin sensitive.
The other choices, Escherichia coli(choice A) and Neisseria gonorrhoeae(choice D), are not encapsulated.

The correct answer is A. This patient has Candida esophagitis. Any time a patient presents with dysphagia or
odontophagia and you note white plaques in the oropharnyx (thrush), you can assume that the Candida is
affecting the esophagus as well. The fact that the patient is an IV drug user makes an opportunistic infection
such as Candida more likely. Now comes the hard part–remembering the morphology of Candida.
Candida appears as budding yeast with pseudohyphae in vivo.
The other answer choices represent the morphology of other important opportunistic fungi:
Cryptococcus are encapsulated yeast (choice B). You should think about Cryptococcus neoformans when
you're presented with an immunocompromised patient with neurological symptoms. The classic clue is the
presence of encapsulated organisms observable in an India ink preparation.
Mucor and Rhizopus are molds with nonseptate hyphae (choice C). You should think about Mucor when you are
presented with a diabetic (especially ketoacidosis) or a leukemic patient with a severe sinus infection.
Aspergillus is a mold with septate hyphae (choice D). In immunocompromised patients, aspergillosis can present
with acute pneumonia, often with cavitation (aspergillomas = fungus balls in the lungs).

The correct answer is A. Anthrax forms extremely stable spores and has, in fact, been encountered in very old
(i.e., deep) dirt in Israeli and Arabian excavation sites. The causative organism, Bacillus anthracis, is found in
many animal species, and humans can acquire the organism either through contact with the animals or from
locally contaminated soil. The pustule described in the question stem is called a "malignant pustule" and may
be accompanied by lymphadenopathy. Most cases remain localized, but death can occur as the result of
complications such as bacteremia, meningitis, and pneumonia. Borrelia burgdorferi(choice B) causes Lyme disease. In the first stage of this disease, there is a localized
expanding erythematous rash rather than an eschar. Furthermore, Lyme disease occurs in the U.S., Europe,
and Asia, corresponding to the distribution of Ixodid ticks, which spread the infection.
Francisella tularensis(choice C) causes tularemia. The organism can persist for weeks to months but does not
form spores that could survive for hundreds or thousands of years. Also, most human cases occur in the
endemic areas of the U.S.
Spirillium minus(choice D) is one of the causes of rat-bite fever. There is no indication the man was bitten by a
rat.
Yersinia pestis(choice E) causes bubonic plague. This organism does not form stable spores that could survive
for extended periods of time.

The correct answer is A. The rare, but life-threatening, meningoencephalitis caused by Naegleria fowleri
apparently ensues when the organism enters the body through the nasal mucosa and crosses the cribriform
plate to enter the brain via the olfactory nerves. It has been speculated that trauma to the nose during diving
predisposes to this infection.
The lacrimal bone (choice B) of the medial aspect of the orbit does not appear to be involved in amoebic
infection. The mouth and its associated bones, including the mandible (choice C), do not appear to be a route of infection
for this organism.
The nasal bones (choice D), located on the superior surface of the nose, do not appear to be a route of
infection for this organism.
The zygomatic bone (choice E), which forms the cheekbone, does not appear to be a route of infection for this
organism.

The correct answer is E. This is a classic example of an infection with herpes simplex virus (probably type 2).
This agent causes lytic infections in mucoepithelial cells. It is then retrogradely transported into neurons of the
sacral ganglia, where it lays dormant during the asymptomatic phase of the disease.
Fibroblasts (choice A) may be infected by cytomegalovirus, another herpesvirus, but this produces a distinctive
mononucleosis-like syndrome in most normal individuals.
Lymphocytes (choice B) and macrophages (choice C) may be infected by herpes simplex type 2 but are not the
site of residence of the virus during quiescent periods.
Infection of mucoepithelial cells (choice D) by herpes simplex produces vesicular-like lesions on the mucous
membranes during symptomatic periods.
Neurons of the trigeminal ganglia (choice F) may be latently infected with herpes simplex type 1. This agent is a
possible cause of genital ulcers, but is usually associated with perioral lesions (cold sores).

The correct answer is C.Enterobius vermicularis (pinworm) inhabits the large intestine, and the gravid females
migrate at night into the perianal region to lay eggs, which can be identified by placing cellophane tape on the
perianal skin and then looking at the tape with a microscope. Enterobiasis is characterized by extreme pruritus
and is very contagious, transmitted by ingestion of the eggs of the organism. The infection is easily treated with
a single dose of mebendazole.
Ancylostoma duodenale(choice A) and Necator americanus(choice D) are hookworms, and typically cause
anemia.
Ascaris lumbricoides(choice B) is a large, intestinal roundworm that is very common, especially in the tropics.
Infection may be asymptomatic or can cause abdominal symptoms.
Trichuris trichiura(choice E) is the whipworm, which can cause rectal prolapse with heavy infestations.

The correct answer is E. The rash described is that of secondary syphilis, caused by Treponema pallidum.
Involvement of palms and soles by a rash is unusual, and secondary syphilis should come to mind. Not all
patients with secondary syphilis have a severe form of the rash, and consequentially some cases are missed.
Primary syphilis takes the form of a painless, button-like mass called chancres. Tertiary syphilis, which is now
rare, has a propensity for involving the aorta and central nervous system and can also cause "gummas"
(granulomatous-like lesions) in many sites, notably including liver and bone. Herpes simplex I (choice A) usually causes perioral vesicular lesions.
Herpes simplex II (choice B) usually causes genital vesicular lesions.
HIV (choice C) does not itself cause a rash, although co-infection with other organisms can result in a rash. Neisseria gonorrhoeae(choice D) does not typically cause a rash.

The correct answer is B. The patient probably has hepatic amebiasis, which is a life-threatening complication of
intestinal infection with Entamoeba histolytica.E. histolytica is transmitted via the fecal-oral route. Intestinal
colonization (which may be asymptomatic) always precedes infection of the liver. The abscesses generally
contain necrotic debris, with amoebae located along the edges of the abscess.
Ascaris lumbricoides(choice A) can cause intestinal obstruction, but does not usually cause hepatic abscesses.
Enterobius vermicularis(choice C) is the pinworm, which inhabits the rectum.
Bloody diarrhea can also be seen with some strains of Salmonella (choice D) and Shigella (choice E), but these organisms do not usually cause hepatic abscesses. Bacterial causes of hepatic abscesses include E. coli, Klebsiella, Streptococcus, Staphylococcus, Bacteroides, and Pseudomonas.

The correct answer is E. The findings described are those of tabes dorsalis, a form of tertiary syphilis caused
by Treponema pallidum. Tabes dorsalis and other forms of tertiary syphilis are now uncommon in this country,
possibly because the common use of antibiotics may "treat" many unsuspected cases of syphilis.
Haemophilus influenzae(choice A) and Neisseria meningitidis(choice D) can cause meningitis.
Herpes simplex I (choice B) can cause an encephalitis that typically involves the frontal and temporal lobes.
Neisseria gonorrhoeae(choice C) causes gonorrhea, which usually does not involve the CNS.

The correct answer is B. The three major causes of neonatal meningitis are group B streptococci
(Streptococcus agalactiae; choice D), Escherichia coli(choice A), and Listeria monocytogenes. All can be found
in the vaginal tract of normal women and may contaminate the infant during passage through the birth canal.
They colonize the upper respiratory tract and can cause pneumonia, septicemia, and/or meningitis in the
neonate. They are readily distinguished on morphologic grounds; the streptococci are gram-positive cocci in
chains, E. coli is a gram-negative rod, and L. monocytogenes is a gram-positive pleomorphic rod. There are
other gram-positive rods that resemble Listeria (e.g., the diphtheroid bacilli found in the upper respiratory tract
and on the skin), hence a motility test is done to confirm the identification. L. monocytogenes is motile at room
temperature but not when grown at 37 degrees C. L. monocytogenes is also associated with drinking
unpasteurized milk.
Neisseria meningitidis(choice C) is the most common cause of meningitis in school-age children and young
adults. It is a fastidious, non-motile, gram-negative diplococcus that would be a very rare cause of meningeal
disease in very young patients, such as this one.
Streptococcus pneumoniae(choice E) is a gram-positive coccus that grows in pairs and short chains. It is the
number one cause of pneumonia, septicemia, and meningitis in the elderly. There is a vaccine composed of the
capsular carbohydrate of 23 serotypes of this organism that is routinely given to individuals over the age of 60,
as well as to individuals with splenic abnormalities (e.g., sickle cell disease) who are at increased risk for the
development of pneumococcal sepsis.

The correct answer is E. The syndrome represented by the clinical vignette is infectious mononucleosis.
Epstein-Barr virus (EBV) is the usual cause of heterophile-positive infectious mononucleosis; cytomegalovirus responsible for a minority of cases. Rarely, splenic rupture requiring splenectomy can result from splenomegaly
and capsular swelling, usually occurring during the 2nd and 3rd weeks of the illness.
Acute cholecystitis (choice A) is not associated with infectious mononucleosis. The most frequently isolated
pathogens are E. coli, Klebsiella spp., group D Streptococcus, Staphylococcus spp., and Clostridium spp.
Ascending cholangitis (choice B) is not associated with infectious mononucleosis. Cholangitis usually presents
with biliary colic, jaundice, and spiking fever with chills (Charcot's triad). Blood cultures are usually positive (E.
coli is a common isolate), with an accompanying leukocytosis. AIDS-related cholangitis has been reported,
presenting with abdominal pain and obstructive liver symptoms. Potential etiologic agents include
Cytomegalovirus, Cryptosporidium parvum, and Microsporidia, including Enterocytozoon cuniculi.
Diarrhea (choice C) is not usually produced by infectious mononucleosis.
Immune complex vasculitis (choice D) is not associated with infectious mononucleosis.

The correct answer is D. The patient is exhibiting the classic signs of acute bacterial endocarditis. In intravenous
drug abusers, Staphylococcus aureus is the most common causative agent, because it is the most common
normal flora organism on the skin of these patients, and choice D is the classic description of this species.
Catalase positivity distinguishes the genus Staphylococcus from Streptococcus, and Staphylococcus aureus is
the only coagulase positive member of its genus.
Antibodies to p24 capsid antigen (choice A) would be likely in an HIV-positive drug abuser, rather than a patient
with endocarditis.
Trichinella spiralis(choice B) can cause splinter hemorrhages during the larval migration period, but would not
be the most likely agent given the other symptoms.
Staphylococcus epidermidis is a catalase-positive, novobiocin-sensitive, gram-positive coccus (choice C). It is
sometimes implicated in subacute bacterial endocarditis, but is less common as a causative agent in drug
abusers than in normal individuals.
Enterococci are gamma-hemolytic, gram-positive cocci that grow on bile-esculin agar (choice E). Enterococci
can cause subacute bacterial endocarditis, chiefly after urologic instrumentation in men, rather than in
association with intravenous drug abuse.
Streptococcus viridans is an alpha-hemolytic, optochin-resistant, gram-positive coccus (choice F). It is implicated
in subacute bacterial endocarditis after oral or dental treatments but would not be the most likely agent, given
the acute nature of this patient's presentation.
Streptococcus pneumoniae is an alpha-hemolytic, optochin-sensitive, gram-positive coccus (choice G) that
produces cough and chest pain, but would be an unlikely cause of bacterial endocarditis.

The correct answer is B. Although it is far better to rely on microbiology lab results than physical exam and
history, some classic clues for an infection with Klebsiella pneumoniae are a patient who is an elderly alcoholic
or diabetic, "currant-jelly" sputum (containing blood clots), and lobar pneumonia.
Hemophilus influenzae(choice A) is a frequent cause of community-acquired pneumonia, but does not
classically produce lobar pneumonia or currant-jelly sputum.
Pseudomonas aeruginosa(choice C) classically produces greenish sputum, and is associated with cystic fibrosis
rather than alcoholism.
Staphylococcus aureus(choice D) often produces pulmonary abscess, but may also produce an ordinary
bronchopneumonia.
Streptococcus pneumoniae(choice E) is a classic cause of lobar pneumonia, but does not produce currant-jelly
sputum or show a particular predilection for alcoholics.

The correct answer is F.Pseudomonas aeruginosa is a gram-negative rod. It can easily be distinguished from
the family Enterobacteriaceae because Pseudomonas is oxidase positive. It is an opportunistic pathogen that
has an increased chance of causing urinary tract infections in patients with indwelling catheters, or who are on
antibiotics.
Candida albicans(choice A) is a yeast that can cause urinary tract infections in poorly controlled diabetics,
because glucose in the urine enhances its growth.
Enterococcus faecalis(choice B) is a gram-positive coccus that commonly causes urinary tract infections in
elderly men with prostate problems.
Escherichia coli(choice C) is a lactose-positive, oxidase-negative, gram-negative rod that is the most common
cause of community acquired urinary tract infections.
Klebsiella pneumoniae(choice D) is a lactose-positive, oxidase-negative, gram-negative rod that can cause
urinary tract infections in poorly controlled diabetics because glucose in the urine enhances its growth.
Proteus mirabilis(choice E) is a gram-negative rod that is a member of family Enterobacteriaceae. It is
lactose-negative, but contains urease, which splits urea to yield ammonia, which in turn, raises the pH of the
urine, creating a more hospitable environment for the organism. Patients with stones are at increased risk for
this organism because it is able to hide in the stones (conversely, Proteus predisposes for the development of
stones).
Staphylococcus saprophyticus(choice G) is a catalase-positive, coagulase-negative, gram-positive coccus that
causes urinary tract infections in young women.

The correct answer is D. This patient has developed hemolytic-uremic syndrome (HUS), a complication of the
Shiga toxin or Shiga-like toxin: exotoxins released by Shigella species and the enterohemorrhagic E.coli. HUS in
children usually develops after a gastrointestinal or flu-like illness, and is characterized by bleeding, oliguria,
hematuria and microangiopathic hemolytic anemia. Presumably the Shiga toxin is toxic to the microvasculature,
producing microthrombi that consume platelets and RBCs, and may fragment the red cell membrane.
The incorrect choices are all bacteria which may produce an enterocolitis, but do not elicit HUS.
A long-term consequence of Campylobacter (choice A) infection is a reactive arthritis or full-blown Reiter's
syndrome.
Clostridial enterocolitis is produced by Clostridium difficile(choice B), a normal inhabitant of the gut that
produces pseudomembranous colitis when other gut flora are suppressed by treatment with antibiotics.
In the United States, Salmonella infections (choice C) are almost all non-typhoid inflammatory diarrhea,
producing a simple enterocolitis that may proceed to sepsis in some cases. Typhoid fever (produced by
Salmonella typhi and S. paratyphi) produces a protracted illness that progresses over several weeks and
includes rash and very high fevers, but not HUS.
Vibrio (choice E) infections produce copious amounts of watery diarrhea, and the major risk of cholera and
other Vibrio enteritides is shock due to hypovolemia or electrolyte loss.

The correct answer is E. Urinary tract infections are the most common bacterial infections encountered during
pregnancy, and Escherichia coli is the most commonly isolated organism. 70% of cases in the U.S. are caused
by P pili-positive strains.
Bundle-forming pili (choice A) are found in enteroaggregative E. coli (EAEC).
GVVPQ fimbriae (choice B) are found in EAEC.
Heat labile toxins (choice C) are pathogenic factors in enterotoxic strains (ETEC).
Heat stable toxins (choice D) are pathogenic factors in ETEC or EAEC.
Type 1 pili (choice F) are a major pathogenic factor in ETEC.

The correct answer is E.Mycobacterium leprae is endemic to parts of Africa, Asia, and South America.
Tuberculoid leprosy is an indolent disease, typically affecting cooler parts of the body, such as the nose and
ears, producing dermal granulomas with very rare, acid-fast bacilli and damage to peripheral nerves. This is in
marked contrast to lepromatous leprosy, which is progressive and invasive and generally characterized by the
presence of numerous acid-fast bacteria in a histiocytic, but non-granulomatous tissue response. M. leprae has
not been grown in any culture medium.
Cutaneous leishmaniasis (choice A) is due to Leishmania species that show a worldwide distribution. The
infection is transmitted through a sandfly bite, and the skin lesion is typically ulcerated. Histology shows
intracellular parasites within the dermis and epidermis. Granulomas are not formed.
Onchocerciasis (choice B), or river blindness, is a roundworm infection transmitted by black flies of Africa and
South America. The microfilaria grow at the site of inoculation, and cause an inflamed subcutaneous nodule.
The organism is seen on tissue sections.
Rhinoscleroma (choice C) is a destructive granulomatous infection of the nasopharynx caused by Klebsiella
rhinoscleromatis. Gram-negative rods can be cultured from the lesions.
Smallpox (choice D), the infection caused by the variola virus (a DNA poxvirus), produces malaise, headaches,
and a macular/pustular rash involving the face and distal extremities. Smallpox has been eradicated through
worldwide vaccination.

The correct answer is C. Carcinomas of the bladder and renal pelvis are usually transitional cell (choices D and
E) carcinomas. However, Schistosoma haematobium infection (where Schistosomes lay eggs in the veins near
the bladder, thereby inducing a marked inflammatory response) is associated with squamous metaplasia and squamous cell carci noma of the bladder. Some authors have suggested that medications used to kill the worms may contribute to the etiology. Adenocarcinomas of the renal pelvis and bladder (choices A and B) are rare.

The correct answer is A. The combination of a one week incubation period, the Eastern seaboard clue, and the
intra-erythrocyte parasites strongly suggest Babesia microti as the infecting organism. The clinical disease is
called babesiosis. The infecting protozoan is related to Plasmodium and is transmitted by the bite of the Ixodes
dammini tick. Occasional cases have been transmitted by blood transfusions. Most patients are asymptomatic;
symptomatic cases in reasonably healthy individuals have the features listed in the question stem. Rare severe
cases, which may be fatal, may develop in severely debilitated or asplenic individuals and can be accompanied
by severe hemolysis (up to 30% of RBCs may have the parasites) with subsequent hemoglobinuria, hemolysis,
and renal failure.
Leishmania donovania(choice B) is an intracellular tissue protozoan, not a blood protozoan.
Plasmodium falciparum(choice C) and Plasmodium vivax(choice D) are malarial parasites not encountered on the Eastern seaboard. Trypanosoma cruzi(choice E) is an intracellular tissue protozoan, and does not infect blood cells.

The correct answer is C. Rotavirus is the most common cause of gastroenteritis in children between 3 months
and 2 years of age. It is most prevalent in the winter. Rotavirus, one of the reoviruses, looks like a wheel (which
ROTAtes) and possesses a double-shelled icosahedral capsid with no envelope. Its genome consists of 11
segments of double-stranded RNA.
A complex double-stranded DNA genome (choice A) is found in enteric adenoviruses, the third most common
cause of gastroenteritis in infants and children. This organism possesses an icosahedral nucleocapsid.
A partially double-stranded circular DNA genome (choice B) is characteristic of hepatitis B. Its envelope contains
surface antigen (HBsAg). Its capsid is icosahedral and contains the genome along with DNA-dependent DNA
polymerase, which also has reverse transcriptase activity.
A single-stranded circular RNA genome (choice D) is characteristic of hepatitis D virus. Its envelope consists of
HBsAg. The virus is defective and is able to replicate only in cells infected with hepatitis B.
A single-stranded RNA genome (choice E) is characteristic of several viruses that cause gastroenteritis in
children, including astrovirus and Norwalk virus. Astrovirus is the second most common cause of viral
gastroenteritis in young children. Three structural proteins form its capsid. Norwalk virus is the most common
cause of gastroenteritis outbreaks in older children and adults. Its capsid consists of one structural protein.

The correct answer is E.Yersinia enterocolitica is an important cause of “mini-epidemics” of
pediatric diarrhea. Adults can also be affected, but less commonly than children. Some diarrheal cases are
severe (and occasionally fatal) and may be complicated by severe dysentery, appendicitis, or chronic relapsing
ileocolitis that may require antibiotics to shorten the course.
Helicobacter jejuni(choice A) is an important cause of hospital-acquired diarrhea.
Salmonella typhi(choice B) causes typhoid fever. Shigella species (choice C) causes epidemics of dysentery in military camps. Vibrio cholerae(choice D) causes cholera.

The correct answer is E. The antibiotic property of vancomycin depends upon its ability to bind D-ala-D-ala,
which is vital in the synthesis of peptidoglycan peptide bridges. Vancomycin-resistant enterococci instead utilize
D-lactic acid in their peptide bonds, and thus are resistant to the action of vancomycin.
Enzymatic deactivation of antibiotics is a common mode of resistance to aminoglycosides, chloramphenicol, and
sulfonamides. The most common chemical modifications are acetylation (choice A), adenylation, and
phosphorylation.
Penicillin that has penetrated the cell membrane is kept in place by binding to penicillin binding proteins (PBPs).
Penicillin resistance may be conferred through altered PBP affinity or number (choice B). Vancomycin
resistance does not depend on PBPs.
β-lactamase (choice C) is a bacterial enzyme that inactivates some of the penicillins and cephalosporins,
but has no activity on vancomycin.
Modified ribosomes with decreased antibiotic affinity (choice D) confer resistance to protein synthesis-inhibiting
antibiotics, including tetracycline, minocycline, doxycycline and the macrolide antibiotics. Vancomycin is not a
protein synthesis-inhibiting antibiotic.

The correct answer is D. The woman has developed a pulmonary abscess, as evidenced by the chest
radiograph. Of the organisms listed, Staph. aureus is the most likely cause of bacterial pneumonia complicated
by abscess formation. Bacteria commonly producing pneumonia developing in the context of influenza include
Staphylococcus aureus, Haemophilus influenzae, and Streptococcus pneumoniae, but Streptococcus
pneumoniae is not a frequent cause of lung abscesses.
Candida albicans(choice A) would not be a likely cause of this woman's abscess, or of the pneumonia that
preceded it.
Klebsiella pneumoniae(choice B) is more likely in men who are alcoholic or diabetic, or suffering from chronic
obstructive lung disease.
Pneumocystis carinii(choice C) is associated with pneumonia in immunocompromised hosts, but there is no
indication of immune compromise in this patient.
Streptococcus pneumoniae(choice E) is only rarely associated with lung abscess.

The correct answer is D. The vagina of prepubertal girls and post-menopausal women is colonized by colonic
and skin bacteria, including Staphylococcus epidermidis, which is normally found on the skin.
The vagina of women of child-bearing age tends to be colonized by Lactobacillus (choice B) species, yeasts
such as Candida (choice A), and Streptococcus species (choice E).
The presence of Neisseria (choice C), such as N. gonorrhoeae (the cause of gonorrhea), in the vagina of a 5
year-old strongly suggests sexual abuse.

The correct answer is B. The symptoms suggest infection with Neisseria gonorrhoeae; however, with Gram's
stain results alone, it is not possible to distinguish the gonococcus from normal flora organisms such as
Acinetobacter, unless gram-negative diplococci are found within polymorphonuclear leukocytes. The fastest
and most reliable assay specific for diagnosis of gonorrhea is the use of commercial DNA probes, with results
available in 2-4 hours. Culture on Thayer-Martin medium (choice A) is indeed the choice for culture of N. gonorrhoeae, but would yield
slower results, and therefore be a secondary choice.
Serology (choice C) is not a good choice because N. gonorrhoeae does not have a significant capsule (the
meningococcus does), and furthermore, serological tests for the gonococcus have proven insensitive and
nonspecific.
The germ tube test (choice D) is one of the diagnostic tests for Candida albicans, but this case presentation is
not characteristic of candidiasis.
The RPR (choice E) is a test for reaginic (heterophilic) antibodies formed early in infection with Treponema
pallidum, but this case presentation is not suggestive of syphilis.