USMLE : Microbiology Hardest Test! Trivia Quiz

Explanation
The correct answer is C. This patient is presenting with signs and symptoms of acute infective endocarditis (IE).
The fact that the patient is an intravenous drug user places him at a very high risk for the development of IE.
The factors that determine the clinical presentation of IE include the nature of the organism, the valve infected,
and the route of the infection. Although the clinical characteristics of IE can vary, most patients present with a
febrile illness lasting several days to 2 weeks. This illness is often accompanied by a variety of nonspecific signs
and symptoms such as chills, weakness, dyspnea, cough, arthralgia, diarrhea, and abdominal pain. Heart
murmurs occur in approximately 90% of all patients, but may be absent in patients with right-sided infections.
Other clinical signs include Osler nodules (purplish or erythematous subcutaneous papules or nodules on the
pads of the fingers and toes), Janeway lesions (hemorrhagic painless plaques on the palms and soles),
petechiae (small erythematous painless hemorrhagic lesions that may appear anywhere), and splinter
hemorrhages (thin linear hemorrhages found under the nail beds of fingers and toes). The diagnosis of IE is
dependent on positive blood cultures and echocardiographic evidence of "valvular vegetation" and/or valvular
injury, with echocardiography the preferred method of diagnosis. A past medical history of intravenous drug
abuse contributes to the diagnosis. The initial treatment measures of IE should be aimed at the stabilization of
cardiac and/or respiratory symptoms, if applicable. The second priority is obtaining three blood cultures from
different veins. Once the blood cultures are obtained, empiric antibiotic therapy for Staphylococci, Streptococci,
and Enterococci should be instituted. Since Staphylococcus aureus accounts for most cases of acute IE,
empiric coverage should primarily be directed toward Staphylococcus aureus.
Enterococci(choice A) cause a minority of cases of native valve endocarditis, but are not usually involved in
endocarditis associated with intravenous drug abuse.
Gram-negative organisms such as Pseudomonas aeruginosa(choice B) are rarely the cause of IE.
Streptococcus pneumoniae(choice D) and Streptococcus pyogenes(choice E) are not common causes of IE.

Explanation
The correct answer is A. The patient's presentation is consistent with a simple urinary tract infection; there is a short history of dysuria, increased urinary frequency and the appearance of white blood cells and gram-negative rods in the urine. Urinary tract infections are common in women after they become sexually active. The infection is likely caused by urethral trauma during intercourse, which leads to bacterial contamination of the bladder. Since the majority of these infections are caused by Escherichia coli (a gram-negative rod), the most appropriate therapy would be ampicillin for around 10 days. Ceftriaxone (choice B) is the treatment of choice for uncomplicated infections with N. gonorrhoeae, now that most strains are resistant to penicillin. Intravenous ceftriaxone is a regimen reserved for the treatment of life-threatening infections. Fluconazole (choice C) is indicated for the treatment of vaginal candidiasis. Since there is no vaginal discharge and the patient has gram-negative rods in the urine, a diagnosis of vaginal candidiasis can be excluded.
Gentamicin (choice D) would be an inappropriate choice since the majority of urinary tract infections caused by gram negative rods are sensitive to ampicillin and the potential for toxicity secondary to gentamicin is great. Metronidazole (choice E) is an antibiotic typically used in the treatment of Trichomoniasis, Giardiasis, and Gardnerella, as well as serious infections believed to be caused by anaerobic bacteria. Since there is no vaginal discharge and the patient has gram negative rods in the urine, one can conclude that these infections
are not present and the patient instead has a urinary tract infection.

Explanation
The correct answer is D. Parvovirus B19 causes erythema infectiosum, or Fifth disease. The 5-year-old boy has
the classic "slapped cheek" appearance. Adults typically do not get the facial rash, but have arthralgias and
arthritis. The symmetrical distribution of involved joints is similar to that in rheumatoid arthritis. The onset in
adults is typically 3 to 4 weeks after exposure. Parvovirus infections may persist in immunosuppressed patients,
resulting in red blood cell aplasia.
Listeriosis (choice A) is caused by the gram-positive rod Listeria monocytogenes. Meningitis and bacteremia
are common clinical manifestations. Elderly, neonates, pregnant women, and those taking steroids have the
highest risk for infection.
Lyme disease (choice B) is caused by Borrelia burgdorferi. It is the most common vector-borne disease (Ixodes
ticks) in the U.S. The incidence is highest in the summer and fall. The white-footed mouse and the white-tailed
deer are zoonotic reservoirs. From 3 to 32 days following a tick bite, the patient develops fever,
lymphadenopathy, meningismus, and the characteristic rash (erythema migrans). The rash enlarges and
resolves over 3 to 4 weeks. Sequelae include arthritis, carditis, and neurologic abnormalities.
Mumps (choice C) is caused by a Paramyxovirus. The virus most commonly affects glandular tissue. Parotitis,
pancreatitis, and orchitis are characteristic. Mumps meningoencephalitis is one of the most common viral meningitides. Mumps polyarthritis is most common in men between the ages of 20 and 30 years. Joint symptoms
begin 1 to 2 weeks after the parotitis subsides and large joints are involved.
Reiter's syndrome (choice E) is a seronegative, asymmetric arthropathy predominantly affecting the lower
extremities. It may be triggered by a C. trachomatis infection. In addition to the arthritis, patients may have
urethritis (which is usually due to chlamydia), conjunctivitis, mucocutaneous disease such as balanitis, oral
ulcerations, or keratoderma. Approximately 80% of patients are HLA-B27 positive.

Explanation
The correct answer is C. Spontaneous abortion is a problem associated with Listeriosis, caused by Listeria
monocytogenes. The pattern of abortions was first recognized in herd animals, notably sheep and cattle, and
then listeriosis was later implicated as a cause of spontaneous abortion in pregnant women. Listeriosis can
occur in either epidemic (food-borne or hospital-acquired) forms or may be sporadic (noticed in animal or animal
product handlers). Soft cheeses like Brie are a particularly common source of food-borne listeriosis. The disease
may range in severity from asymptomatic carrier cases, to flu-like illness, to spontaneous abortion or neonatal
death, to fatal illness in children or adults secondary to septicemia or meningoencephalitis. Other localized
infections can also occur, primarily in the immunosuppressed. The treatment of choice is intravenous
administration of ampicillin or penicillin, often in combination with an aminoglycoside.
Trimethoprim-sulfamethoxazole has been used successfully in patients with penicillin allergy.
Borrelia burgdorferi(choice A) causes Lyme disease.
Leptospira interrogans(choice B) causes leptospirosis.
Spirillium minus(choice D) is a cause of rat-bite fever.
Streptobacillus moniliformis(choice E) is a cause of rat-bite fever.

Explanation
The correct answer is A. The patient has myocarditis due to Trypanosoma cruzi. This infectious condition,
known as Chagas disease, is endemic in vast areas of South America and is transmitted from person to person
by triatomids known as "kissing bugs." Experts assess the number of persons with Chagas disease at about 7
million with about 35 million at risk in South America. T. cruzi is an intracellular protozoon that localizes mainly in
the heart and nerve cells of the myenteric plexus, leading to myocarditis and dysmotility of hollow organs, such
the esophagus, colon, and ureter. Cardiac involvement manifests with ventricular dilatation and congestive
heart failure secondary to myocyte necrosis and fibrosis. Intracellular parasites can be visualized in tissue
sections. Chagas disease is a cause of acquired achalasia, in which the distal third of the esophagus dilates
because of loss of its intrinsic innervation. A similar pathologic mechanism accounts for megacolon and
megaureter in Chagas disease.
The remaining choices refer to different infectious conditions that may also involve the myocardium:
Chronic arthritis (choice B) is a manifestation of the chronic stage of Lyme disease, which is caused by Borrelia
burgdorferi and is transmitted to humans by deer ticks. Skin, CNS, and heart are the main targets of this
infection.
Cysts in the brain (cysticerci) (choice C) may develop as a consequence of infestation by the tapeworm Taenia
solium. Humans acquire this parasite by ingesting the eggs from undercooked pork. Cysticercosis may also
affect the heart, skeletal muscle, and skin.
Group B coxsackievirus infections cause pleuritis (choice D) and myocarditis, manifesting with fever, chest pain,
and, if myocarditis is severe, congestive heart failure. As in any form of viral myocarditis, the myocardium is
infiltrated by lymphocytes, but there are no morphologic markers specific for Coxsackievirus infection.
Splenomegaly (choice E), often of massive proportions, is seen in patients with malaria. Plasmodium organisms
can also invade the myocardium, leading to myocarditis.

Explanation
The correct answer is B. The patient's clinical presentation is typical of primary atypical pneumonia. In contrast
to bacterial pneumonia, primary atypical pneumonia presents with the following features: - Caused by M. pneumoniae; less frequently by viruses (influenza, respiratory syncytial virus, adenovirus,
rhinoviruses, rubeola and varicella virus), Chlamydia, or Coxiella burnetii
- Characterized pathologically by interstitial, rather than intra-alveolar, inflammation
- Characterized clinically by nonspecific symptomatology and few "localizing" symptoms
Why is M. pneumoniae, and not influenza virus (choice A) or respiratory syncytial virus (choice D), the cause of
this patient's pneumonia? First, M. pneumoniae infections are often associated with the appearance of cold
agglutinins in the serum, detection of which is diagnostically important. Second, the patient responded quickly to
treatment with erythromycin, an antibiotic effective against M. pneumoniae, but obviously not effective in treating
viral infections.
Pneumocystis carinii(choice C) is a fungal organism causing pneumonia in severely immunocompromised hosts,
especially AIDS patients. P. carinii pneumonia (PCP) is characterized by accumulation of a frothy exudate
containing numerous organisms within alveolar spaces. Also, P. carinii is not sensitive to erythromycin. The drug
of choice for treatment of PCP is trimethoprim-sulfamethoxazole.
Streptococcus pneumoniae(choice E) is the usual causative agent of lobar pneumonia, characterized by consolidation of a single lobe due to intra-alveolar acute inflammatory exudation. Lobar pneumonia is more prevalent in young, healthy individuals, whereas primary atypical pneumonia favors old, debilitated patients. S.pneumoniae is highly sensitive to penicillin.

Explanation
The correct answer is D. Rubella, also called German measles or 3-day measles, is a disease caused by a
Togavirus, which are small, enveloped, single-stranded, (+) linear RNA viruses. Approximately 40% of patients
are asymptomatic or have mild symptoms. In symptomatic patients, the clinical presentation typically consists of
an erythematous rash beginning on the head, which spreads downward to involve the trunk, lasting for
approximately 3 days. In addition to a transient rash, symptoms include fever, posterior cervical
lymphadenopathy, and arthralgias. The greatest danger from rubella is to the fetus. If clinical rubella develops
or seroconversion is demonstrated, there is a high risk of congenital abnormalities or spontaneous abortion.
The risk varies from 40%–60% if infection occurs during the first 2 months of gestation to 10% by the 4th
month. Females of childbearing age should be warned not to become pregnant within 2-3 months from the time
of immunization. Mild arthralgias and other symptoms may develop in 25% of immunized women. Enteroviral
rashes may mimic rubella and rubeola.
Infectious mononucleosis (choice A) is caused by the Epstein-Barr virus, a herpesvirus. Classic findings include
fever, exudative pharyngitis, generalized lymphadenopathy, severe malaise (most common complaint), and
hepatosplenomegaly. A rash is not a characteristic feature unless the patient has been treated with ampicillin.
Lyme disease (choice B) is caused by the spirochete Borrelia burgdorferi. The disease is transmitted by the bite of the tick, Ixodes dammini. Reservoirs in nature include the white-tailed deer and the white-footed mouse. The
initial lesion is an annular rash with central clearing and a raised red border (erythema chronicum migrans) at
the bite site. The rash is warm, but not painful or itchy. Patients also have fever, malaise, myalgias, arthralgias,
headache, generalized lymphadenopathy, and, occasionally, neurologic findings.
Roseola (choice C) is caused by human herpesvirus 6. Other names include exanthem subitum or sixth
disease. Children have a febrile period of 3–5 days with rapid defervescence followed by an
erythematous maculopapular rash lasting 1–3 days.
Rubeola (choice E), or regular measles, is a disease caused by a paramyxovirus. Patients present with an
upper respiratory prodrome and characteristic oral lesions (Koplik's spots) that precede the rash. The
non-pruritic maculopapular rash begins on the face and spreads to the trunk and extremities, including palms
and soles. The incubation period is 10–14 days. Patients also have a posterior cervical
lymphadenopathy. The virus is not associated with risk to a fetus.

Explanation
The correct answer is B. Vitamin B12 deficiency with resulting megaloblastic anemia is specifically associated
with infection with the fish tapeworm, Diphyllobothrium latum, found in Scandinavia and the Great Lakes. The
adult worm attaches to the small intestinal mucosa and releases eggs into the feces. Crustaceans living in
contaminated water ingest free-swimming embryos that hatch from the eggs. Certain species of fish ingest the
crustaceans, then humans acquire the infection by ingesting the undercooked or raw fish. Vitamin B12
deficiency can develop because the tapeworm absorbs vitamin B12 from the intestinal lumen and also interferes
with ileal B12 absorption.
Clonorchis sinensis(choice A) infections produce obstruction of the biliary tract or pancreatic duct and are
associated with an increased risk of cholangiocarcinoma.
Echinococcus granulosus(choice C) larvae infect humans and produce unilocular cystic lesions in the liver.
Echinococcus is prevalent in areas in which dogs are used to help raise livestock.
Taenia saginata is the beef tapeworm (choice D); it causes diarrhea but not megaloblastic anemia.
Taenia solium(choice E) is the pork tapeworm. It occasionally causes diarrhea, but is better known as the cause
of cysticercosis. It does not produce megaloblastic anemia.

Explanation
The correct answer is E. Parvoviruses are small single-stranded DNA viruses, of which only serotype B19 is
pathogenic for humans. This virus causes three distinct syndromes: a childhood febrile rash known as
erythema infectiosum ("Fifth disease"); aplastic crisis in individuals with chronic hemolytic diseases (sickle cell
anemia, thalassemia, etc); and congenital infections that can present as stillbirth, hydrops fetalis (analogous to
severe Rh incompatibility), or severe anemia.
Coxsackie viruses (choice A) usually cause cold-like illness, but can cause herpangina, myocarditis, and
meningitis.
Echoviruses (choice B) can infect a variety of organ systems (GI, CNS, eyes, heart, respiratory, skin), but are
not a cause of aplastic crises.
Hepadnavirus (choice C) is the causative agent of hepatitis B.
Herpes viruses (choice D) cause a variety of acute to chronic infections including herpes simplex types I and II,
chicken pox, chronic herpes zoster, CMV infection, and Epstein Barr virus infections.

Explanation
The correct answer is E. The history is characteristic for Lyme disease, caused by the spirochete Borrelia
burgdorferi. The clues are deer, Connecticut, rash, knees, and acute arthritis followed by chronic arthritis.
Fungal arthritis (choice A) can be caused by coccidioidomycosis, blastomycosis, sporotrichosis, and
candidiasis.
N. gonorrhoeae are gram-negative cocci (choice B) that can cause septic arthritis. Haemophilus (young children) and Salmonella (sickle cell patients) are gram-negative rods (choice C) that can
cause septic arthritis.
Staphylococcus are gram-positive cocci (choice E) that can cause septic arthritis.

Explanation
The correct answer is E. While all five pathogens can cause central nervous system (CNS) manifestations,
toxoplasmosis (caused by Toxoplasma gondii) presents with seizures in 15-25% of cases. The disease is
spread by ingestion of cysts from undercooked meat or from cat feces. Tachyzoites develop from cysts
phagocytized by macrophages, then spread to the brain, muscle, and other tissues, where they encyst and
multiply. Primary CNS lymphoma in AIDS patients can present clinically in nearly the same manner as
toxoplasmosis, but the course is usually much more indolent.
Cryptococcal meningitis (choice A) usually presents as a subacute meningitis with headache, nausea, vomiting,
and confusion. Cranial nerve abnormalities are common with cryptococcal infections.
Herpes simplex encephalitis (choice B) typically has a subacute onset with headache, meningismus, and
personality changes.
Mycobacterium tuberculosis(choice C) is a cause of a basilar meningitis, which can present insidiously with
headache and mental changes over a week or two, or can present acutely as confusion, lethargy, altered
sensorium, and a stiff neck. Cranial nerve palsies, focal cerebral ischemia, and hydrocephalus are
characteristic.
Progressive multifocal leukoencephalopathy (PML) is a demyelinating disease caused by JC virus (choice D), a
human polyomavirus. PML occurs in about 4% of patients with AIDS. The disorder has a slow, insidious onset,
in which altered mental status is not a prominent feature.

Explanation
The correct answer is B. Although bacterial spores are extensively discussed in microbiology courses, you
should be aware that only Bacillus (aerobic) and Clostridium (anaerobic) species produce spores. This means
that the list of diseases related to bacterial spore formation is also small: anthrax (Bacillus anthracis), some
forms of food poisoning (Bacillus cereus), botulism (Clostridium botulinum), tetanus (Clostridium tetani), gas
gangrene (Clostridium perfringens and others), and pseudomembranous colitis (Clostridium difficile).

Explanation
The correct answer is A. Streptococci are usually initially speciated by their hemolytic capacity on sheep blood
agar. Beta-hemolytic streptococci include groups A, B, and D. S. agalactiae is the classic group B
streptococcus. The non beta-hemolytic streptococci consist principally of the pneumococci and the viridans
group.
Both S. agalactiae and pneumococcus have a carbohydrate capsule (choice B), an important virulence factor
and means of subtyping streptococcal species.
None of the streptococci utilize cytochrome enzymes (choice C). They derive all of their energy from the
fermentation of sugars to lactic acid.
Neither pneumococcus nor S. agalactiae can grow in bile (choice D). This ability is specific for the enterococcus
group (group D) of streptococci.
Both pneumococci and S. agalactiae are usually treated with penicillin-type antibiotics, although group B
streptococci require a penicillinase-resistant type such as oxacillin (choice E).

Explanation
The correct answer is B. Proteus species produce urease, which raises the urinary pH and promotes the
production of struvite stones.
E. coli(choice A) are by far the most common cause of urinary tract infections, but are not responsible for the
development of struvite stones.
Pseudomonas aeruginosa(choice C) can also cause urinary tract infections, most commonly in hospitalized or
immunocompromised patients (especially burn patients, patients on immunosuppressives/antimicrobials, and
cystic fibrosis patients). It does not predispose to struvite stones.
Staphylococcus saprophyticus(choice D) is a common cause of urinary tract infections in sexually active young women. Ureaplasma urealyticum(choice E) can produce urease (like Proteus), but they are responsible for urethritis, not stones.

Explanation
The correct answer is E. Tularemia is caused by Francisella tularensis. Fifty percent of the cases have been
reported in Missouri, Arkansas, and Oklahoma. Infection can be transmitted by tick bites or by handling animal
carcasses such as rabbits, squirrels, beavers, muskrats, and deer. The diagnosis is suggested by the history of
exposure, clinical progression, and generalized lymphadenopathy. The ulceroglandular form of tularemia is the
most common clinical presentation.
Anthrax (choice A) is caused by Bacillus anthracis. Typically, infection with B. anthracis causes a localized skin
lesion at the site of inoculation that develops into a black eschar surrounded by draining lymphadenitis.
Woolsorter's disease is the inhalation form of anthrax.
Leptospirosis (choice B) is an infection caused by contact with urine from infected animals such as rats and
dogs. L. interrogans is the most common isolate. The disease is biphasic, with the leptospiremic phase
characterized by abrupt-onset headache (98%), fever, chills, conjunctivitis, severe muscle aches,
gastrointestinal symptoms, changes in sensorium, rash, and hypotension. This phase lasts from 3 to 7 days. The immune stage occurs after a relatively asymptomatic period of 1 to 3 days and is characterized by
recurrence of fever and generalized symptoms. Meningeal symptoms often develop during the immune period.
In more serious cases, hepatic dysfunction and renal failure may develop.
Pasteurella multocida(choice C) is associated with dog and cat bites. It causes a rapidly progressing cellulitis,
bacteremia, and, occasionally, infective endocarditis.
Plague (choice D) is caused by Yersinia pestis. It is enzootic in the southwestern United States. Rats and fleas are the vectors. Clinical presentations include lymphadenopathy with septicemia or pneumonia (which has the highest case-fatality rate).

Explanation
The correct answer is D. The patient has acute prostatitis, which is characterized by fever, chills, and dysuria,
with a swollen, extremely tender prostate on rectal exam. The urine Gram's stain and culture will generally be
positive. The treatment regimen for this bacterial infection is typically a 21-day course of ampicillin, a
fluoroquinolone, or sulfamethoxazole/trimethoprim (SMX-TMP). G-6-PD deficiency is an X-linked recessive
disorder affecting 10% to 15% of American black males. The medications most commonly associated with the
induction of hemolytic anemia in deficient patients are sulfonamides, nitrofurantoin, dapsone, primaquine, and
quinine. The sulfamethoxazole in the SMX-TMP combination is a sulfonamide, and can produce hemolytic anemia
in patients with G-6-PD deficiency.
Ampicillin (choice A) is a broad-spectrum penicillin antibiotic commonly used in the treatment of infections in the genitourinary, respiratory, or GI tracts, as well as in the skin and soft tissues.
Cefaclor (choice B) is a second-generation cephalosporin indicated for a variety of bacterial infections, including those of the respiratory and GI tracts. Ciprofloxacin (choice C) is a fluoroquinolone commonly used in the treatment of serious infections caused by gram-negative organisms. Tetracycline (choice E) is most commonly used in the treatment of acne vulgaris and gonococcal infections.

Explanation
The correct answer is C. The patient has a fever and is extremely ill. The most important clue to the diagnosis is
the presence of bulky vegetations on the tricuspid valve, indicating that he has infective endocarditis. On the
basis of the rapid clinical course, this is likely a case of acute infective endocarditis. The diagnosis of this
condition must be confirmed by blood cultures, which are also necessary to determine bacterial antibiotic
sensitivity. S. aureus (commonly present on the skin) is the most frequent etiologic agent of infective
endocarditis in intravenous drug abusers. It commonly affects the tricuspid valve. Because of its high virulence,
S. aureus-related endocarditis follows an acute course and may lead to death within a few days.
The causative agents of infective endocarditis differ depending on host factors. Fungal organisms, such as
Candida albicans(choice A), may cause infective endocarditis in severely immunosuppressed patients, such as
those with AIDS.
A minority of cases of infective endocarditis are caused by a number of normal commensals in the oral cavity, i.e., the "HACEK" group: Hemophilus(choice B), Actinobacillus, Cardiobacterium, Eikenella, and Kingella. S. epidermidis(choice D) and other coagulase-negative staphylococci tend to produce endocarditis in recipients of prosthetic valves. Viridans streptococci (choice E) are the most frequent agents causing endocarditis in previously abnormal valves, such as those damaged by rheumatic disease, or congenitally abnormal valves. Coagulase-negative staphylococci and viridans (α-hemolytic) streptococci are less virulent than S. aureus and are thus associated with a subacute (more prolonged) clinical course and a better prognosis.

Explanation
The correct answer is A. Essentially, this question tests your understanding of MacConkey's agar and your
knowledge of the biochemical characteristics of the Enterobacteriaceae. MacConkey's agar is used to identify
organisms that ferment lactose, and consequently grow as pink colonies. Escherichia coli is one of several
lactose-fermenting Enterobacteria, along with Serratia, Klebsiella, and Citrobacter species. Neonatal pneumonia
due to aspirated E. coli during delivery is not uncommon, and it should be treated with a third-generation cephalosporin and/or aminoglycoside.
Proteus species (choice B) are motile, non-lactose fermenting bacteria that characteristically swarm on an agar
plate. Proteus infections generally affect the genitourinary tract, but Proteus is an important nosocomial
pathogen.
Pseudomonas species (choice C) are normally widespread in nature, but may form a small portion of the bowel
flora. Pseudomonas utilizes cytochrome oxidase for energy production; it does not ferment any sugars.
Salmonella species (choice D) are non-lactose fermenters that may produce inflammatory diarrheas. They are
normally distinguished from the other Enterobacteriaceae by their ability to produce acid and gas from
dextrose-containing media.
Shigella species (choice E) are non-motile, non-lactose fermenting bacteria that live exclusively in the
gastrointestinal tract of primates. They typically produce dysentery.

Explanation
The correct answer is E. Take rashes involving the palms and soles (otherwise unusual sites) very seriously:
only a small number of infections can cause this pattern, including Rocky Mountain spotted fever,
meningococcemia, and secondary syphilis. Rocky Mountain spotted fever is caused by the rickettsia R.
rickettsii, and is found throughout the United States, particularly in south central and eastern portions (not the
Rocky Mountains). 3-12 days after a tick bite, patients develop malaise, frontal headache, and fever. Several
days later, the rash described in the question stem develops. Other manifestations can include
hepatosplenomegaly, thrombocytopenia, and (potentially fatal) disseminated intravascular coagulation.
Chickenpox (choice A), or varicella, is characterized by maculopapules that evolve into vesicles over hours to
days, then eventually form crusts. Typical lesions first appear on the trunk and face and rapidly spread to
involve other areas.
The maculopapular rash of German measles (choice B), or rubella, usually begins on the face, then spreads
down the body.
Although the maculopapular rash of measles (choice C), or rubeola, can include the palms and soles, it typically
begins along the hairline in frontal and temporal regions, then spreads down the trunk to the limbs.
Mumps (choice D) is characterized by fever, malaise, parotitis, and orchitis, but not a rash.

Explanation
The correct answer is E. For many years, Whipple's disease was suspected of having a bacterial etiology
because bacterial forms could be seen on electron microscopy. However, the identity of the causative agent
remained elusive. It has recently been found to be a bacterium which is now named Tropheryma whippelii.
Clostridium difficile(choice A) causes pseudomembranous colitis, generally after antibiotic administration.
Enterotoxigenic E. coli(choice B) is associated with tropical sprue.
Isospora bella(choice C) is a cause of diarrhea in AIDS patients.
Salmonella sp.(choice D) can cause diarrheal illness after ingestion of contaminated poultry or beef.

Explanation
The correct answer is A.Bartonella henselae is a gram-negative bacillus that is the causative agent of cat
scratch disease. Cats, especially kittens, harbor the organism. It is usually inoculated through an easily
forgotten scratch. A pustule forms at the site within days to weeks, accompanied by tender regional
adenopathy. Microscopically, granulomas with central necrosis and organisms are observed. Normally,
treatment is not required because this disease is benign and self-limited, but immunosuppressed and pregnant
patients are more susceptible to dissemination. Treatment is with ciprofloxacin or doxycycline. Aminoglycosides
or erythromycin have also been employed. Coccidioidomycosis is a disease caused by the fungus C. immitis(choice B). Spores of the organism are
inhaled, usually from the soil. These spores enlarge to sporangia, which burst and spread. The disease is
characterized by pulmonary symptoms, but is usually self-limited. This organism is common in arid desert states
like Arizona, and can affect people after brief visits.
Cryptococcus neoformans(choice C) is a fungus found worldwide. The main reservoir is pigeon droppings. The
small cryptococci are inhaled into the lungs, but do not usually cause disease in immunocompetent individuals.
In the immunosuppressed patient, Cryptococcus can cause pulmonary and CNS disease, especially meningitis.
Histoplasma capsulatum(choice D) is an infection acquired by breathing dust that contains bird/bat droppings
that contain spores of the organism. The primary infection is in the lungs, and may be latent, or brief and
self-limited. Symptoms include fever, cough and sweats. In the United States, it is common in the
Ohio-Mississippi valley.
Pasteurella multocida(choice E) is a gram-negative rod spread via dog and cat bites. The infection is
characterized by rapidly evolving cellulitis starting at the site of the bite. There was no indication in this patient of an animal bite.

Explanation
The correct answer is B. The causative organism is Neisseria gonorrhoeae, which is a sexually transmitted
gram-negative coccus that can live in phagocytic vacuoles in neutrophils and macrophages. The organism is
the most common cause of septic arthritis in otherwise healthy, sexually active adults.
Hyphae and spores (choice A) would be features of Candida, which usually causes arthritis as a rare
complication of systemic candidiasis in immunocompromised patients.
The Chlamydia are intracellular organisms that cannot make ATP (choice C) and do not usually cause septic
arthritis.
Mycoplasma do not have true cell walls (choice D) and do not usually cause septic arthritis.
Syphilis is caused by a spirochete that stains with silver stains (choice E). Secondary syphilis can (uncommonly) cause an acute arthritis, but the interval between acquiring the infection and developing the arthritis is usually months to years.

Explanation
The correct answer is C. The clinically suspected diagnosis is meningitis, which is confirmed by the abnormal
cerebrospinal fluid. The markedly increased lymphocytes suggests acute lymphocytic meningitis, which is
distinguished from acute pyogenic meningitis (increased neutrophils as well as lymphocytes). Acute lymphocytic
meningitis is usually viral in origin. Among the many viruses that have been implicated, mumps, herpes,
Epstein-Barr, echovirus, and Coxsackie virus are the most common.
Escherichia coli(choice A) and Haemophilus influenzae(choice B) cause acute pyogenic meningitis.
Mycobacterium tuberculosis(choice D) and Treponema pallidum(choice E) cause chronic meningitis

Explanation
The correct answer is C.Yersinia enterocolitica is the pathogen producing this clinical syndrome. Yersinia is
transmitted to patients via the oral route, via contaminated blood products, or by cutaneous inoculation.
Patients developing iron overload because of multiple transfusions (i.e., thalassemia patients) are at increased
risk of Yersinia infections because some strains are unable to synthesize bacterial iron chelators called
siderophores. They can, however, use host-chelated iron stores or the drug deferoxamine (a siderophore
produced by Streptomyces pilosus). Yersinia is associated with reactive arthritis following an infection by an
enteropathogenic organism. Most patients who develop arthritis express HLA-B27.
A history of asthma (choice A) is not associated with Yersinia infections.
Deficiency in C1 esterase inhibitor activity (choice B) produces the syndrome of angioedema. This is an
autosomal dominant trait associated with a deficiency of the serum inhibitor of the activated first complement
component. The patients have multiple episodes of edema, affecting skin and mucosal surfaces such as the
larynx and the GI tract.
Leukopenia (choice D) is not associated with the clinical scenario in the question. Normal-to-elevated leukocyte
counts are the rule in Yersinia infection. Serological positivity for toxoplasmosis (choice E) is not suggested because toxoplasmosis produces a different
clinical picture. Toxoplasmosis can be acquired transplacentally with infected infants exhibiting a wide array of
symptoms depending on when the mother was infected during gestation. Toxoplasmosis is also seen in
immunocompromised patients such as those with AIDS. In this group of patients, the main organ system
involved is the central nervous system.

Explanation
The correct answer is E. The patient is suffering from conjunctivitis caused by Hemophilus, a fastidious
organism requiring factors V (nicotinamide adenine dinucleotide) and X (hematin), which must be released from
whole blood preparations to be accessible to the organism. This can be done by gentle heating, which lyses the
red blood cells, or by co-culture with Staphylococcus aureus, which is beta hemolytic and lyses the red blood
cells to provide the factors Hemophilus needs for growth. This is called the satellite phenomenon.
Antibiotics (choice A) would not be necessary since, with proper culture techniques, no normal flora should be
available to confuse the diagnosis from this site.
Cold temperature (choice B) would not be necessary because Hemophilus grows at normal body temperature.
Listeria is the genus commonly mentioned for which a cold temperature is necessary.
Cholesterol (choice C) is necessary for the growth of Mycobacteria, but not Hemophilus.
Low oxygen (choice D) tension is necessary for the growth of the microaerophiles (such as Campylobacter,
Helicobacter, and Borrelia), but is not a condition for growth of Hemophilus.