USMLE Microbiology Part 1

The correct answer is A. The baby has infant botulism (floppy baby syndrome), which is due to germination of
Clostridium botulinum spores (found in honey) in the baby's gastrointestinal tract. Patients improve when honey
is removed from the diet. This disorder is most common in children under the age of six months; older children
and adults do not appear to be vulnerable to this form of botulism, but are susceptible to botulism caused by
ingestion of preformed toxin.
Clostridium difficile(choice B) causes pseudomembranous colitis, especially after antibiotic therapy.
Clostridium perfringens(choice C) causes gas gangrene and gastroenteritis, and it is not associated with
ingestion of honey.
Clostridium tetani (choice D) causes tetanus, and does not cause a food-borne illness in infants.
Corynebacterium diphtheriae(choice E) causes diphtheria in susceptible individuals.

The correct answer is A. This patient has the classic symptoms of cat scratch disease caused by the bacillus
Bartonella henselae. The disease is self-limited with the onset of symptoms occurring 3-10 days following an
inoculating scratch. The organism can be isolated from kittens, typically less than 1 year of age, or from fleas. A
history of a new kitten in the house and the papule at the site of a scratch with regional painful adenopathy
defines the classic scenario.
Chickens (choice B) can harbor Salmonella spp. producing a gastroenteritis or enterocolitis. Chicken guano is
also a favorable environment for the fungus Histoplasma capsulatum. The mycelial phase thrives in the rich soil.
The human disease is a granulomatous infection involving the lungs and mimicking tuberculosis.
Dogs or puppies (choice C) carry Capnocytophaga canimorsus as part of the normal flora of the oral cavity.Infections from licking or biting range from a self- limited cellulitis to fatal septicemia. Patients at risk for more
severe infections are those with asplenia, alcoholism, or hematologic malignancies. This organism is also
associated with cat bites, but the patient develops cellulitis and fulminant septicemia, especially in asplenic
patients. Pasteurella multocida is another pathogen that colonizes the nasopharynx and gastrointestinal tract of
cats and dogs. Cats have the highest rate of colonization (50-90%), followed by dogs (50%), swine (50%), and
rats (14%). P. multocida most commonly causes a localized soft tissue infection or cellulitis after an animal bite,
but systemic symptoms may be present in about 40% of the cases. These symptoms include osteomyelitis,
septic arthritis, or tenosynovitis.
Horses (choice D) and horse manure have been associated with a pulmonary opportunistic infection with
cavitation caused by Rhodococcus equi that resembles tuberculosis in immunocompromised patients.
Burkholderia mallei (the cause of glanders) is characterized by non-caseating granulomatous abscesses of
skin, lymphadenopathy, and pronounced involvement of the lungs.
Parrots (choice E) are associated with psittacosis caused by Chlamydia psittaci. Psittacosis is associated with a
dry, hacking cough productive of scant sputum, an interstitial infiltrate in the lungs, severe headache, and
myalgias. A pale macular rash is also seen.

The correct answer is A. The patient has Burkitt's lymphoma. This type of lymphoma is a high-grade B-cell
lymphoma that occurs in endemic form in Africa (it is the most common neoplasm in children in an equatorial
belt that includes Africa and New Guinea) and sporadically in the United States and Europe. The sporadic form
is often in an abdominal site and occurs in young adults. The African form of Burkitt's lymphoma has been
strongly associated with antibodies directed against Epstein-Barr virus; the association is weaker in sporadic
cases. A characteristic translocation, t(8;14) (q24.l3;q32.33) has been described.
Hepatitis B (choice B) is associated with hepatocellular carcinoma. t(9;22) is the Philadelphia chromosome,
which is seen in some cases of CML and AML.
Herpesvirus (choice C) does not have a strong tumor association, although a link to cervical cancer has
intermittently been proposed. CD5 is a marker seen in small lymphocytic and mantle cell lymphomas.
HIV (choice D) is linked to Kaposi's sarcoma (and AIDS). Some patients also develop primary lymphomas (not
usually Burkitt's). CD4 is a marker for helper T cells and some T cell lymphomas.
Human papillomavirus (choice E) is linked with common warts, genital condylomata, and genital cancers. t(2;5)
is linked to anaplastic large cell lymphoma.

The correct answer is A. Epiglottitis is the most common disease of the upper respiratory tract produced by
Haemophilus influenzae type b, a gram-negative encapsulated rod. It is also a common cause of otitis media in
children and may cause bronchitis, bronchiolitis, and pneumonia in adults. The incidence of serious disease
caused by Haemophilus influenzae type b has decreased greatly with the introduction of an effective vaccine.
The vaccine is composed of the H. influenzae type b capsular polysaccharides coupled to a carrier molecule,
given to children between 2 and 15 months of age. The patient had not received the Hib conjugate vaccine and
therefore was susceptible to this organism.
Klebsiella pneumoniae(choice B) causes pneumonia and pulmonary abscesses, but is not considered to be a
pathogen in the upper respiratory tract.
Legionella pneumophila(choice C) causes pneumonia in man. The disease may be mild (an atypical
pneumonia) or a fulminating disease with a high mortality (30%).
Mycoplasma pneumoniae(choice D) causes community-acquired atypical pneumonia. It is the most common
cause of pneumonia in young adults.
Streptococcus pyogenes(choice E) is the most common cause of pharyngitis; however, this patient's
presentation strongly suggests epiglottitis.

The correct answer is A. The patient has a severe, potentially fatal, pneumonia with prominent systemic
symptoms. Culture on BCYE is the specific clue that the organism is Legionella pneumophila. The disease is
respiratory Legionellosis, also known as Legionnaire's disease, because the disease was first described when it
occurred in epidemic form following an American Legion convention at a Philadelphia hotel. Patients tend to be
older (40-70 years old) and may have risk factors including cigarette use, alcoholism, diabetes, chronic illness,
or immunosuppressive therapy.
Listeria monocytogenes(choice B) causes listeriosis and is not a notable cause of pneumonia.
Spirillium minus (choice C) is a cause of rat-bite fever and is not a notable cause of pneumonia.
Staphylococcus aureus(choice D) can cause pneumonia, but is easily cultured on routine media.
Streptococcus pneumoniae(choice E) can cause pneumonia, but is easily cultured on routine media.

The correct answer is A. The disease is chlamydial cervicitis. This venereally transmitted infection is usually
suspected after treatment for gonorrhea fails to relieve symptoms. However, a few cases are picked up when
cytoplasmic inclusions composed of aggregates of chlamydia are identified on Pap smears. Confirmation can
be made with fluorescent antibodies that pick up both the aggregates and individual bacteria known as
"elementary bodies." The organisms are obligate intracellular parasites because they have lost the ability to
synthesize ATP. Although some authors consider them to be "bacteria," others do not, since this is such a
fundamental difference between the chlamydia and free living bacteria.
Failure to synthesize cholesterol (choice B) is not usually cited as a problem specific to any type of organism,
although viruses are not able to do so without using host machinery.
Some viruses cannot synthesize DNA (choice C), but the disease in question is not caused by a virus.
Protein synthesis (choice D) by viruses requires host ribosomes.
Some viruses cannot synthesize RNA (choice E), but the disease in question is not caused by a virus.

The correct answer is C. Gram-positive cocci in clusters are staphylococci. Staphylococcus aureus is a common
pathogen that should not be considered normal skin flora. Other staphylococci, especially Staphylococcus
epidermidis, may contaminate blood cultures and can be differentiated from Staphylococcus aureus by the
coagulase test. S. aureus is the only coagulase-positive staphylococcus.
Bacitracin sensitivity differentiates Streptococcus pyogenes from the other beta-hemolytic streptococci, which
are bacitracin resistant (choice A).
The catalase test (choice B) is used to differentiate streptococci from staphylococci. Staphylococci produce
catalase and can generate oxygen bubbles in hydrogen peroxide, whereas streptococci cannot.
Novobiocin resistance (choice D) differentiates the coagulase-negative staphylococci into S. epidermidis
(novobiocin sensitive) and S. saprophyticus (novobiocin resistant).
Optochin resistance (choice E) differentiates the major pathogenic alpha-hemolytic streptococci. S. pneumoniae
is optochin- and bile-sensitive whereas S. viridans is resistant to both optochin and bile.

The correct answer is E. This is the classic presentation of a psoas abscess. This clinical entity was formerly a
fairly common complication of vertebral tuberculosis, but is now rare in clinical practice in this country. The
psoas muscle is covered by a fibrous sheath known as the psoas fascia. This sheath is open superiorly,
permitting an infection involving the soft tissues around the spine to enter the sheath, then track down to the
groin.
The adductor longus (choice A) is a muscle of the anterior thigh, and is not related to the lumbar portion of the back.
The gluteus maximus (choice B) gluteus minimus (choice C) and piriformis (choice D) are muscles of the
buttock with no relationship to the groin.

The correct answer is A. The most common agents causing congenital infection in the United States are
cytomegalovirus, herpes simplex virus, and Treponema pallidum. Congenital cytomegalovirus infection, which
occurs in 1 to 2% of all live births in the U.S., results from transplacental acquisition of a primary or recurrent
maternal infection. Most infections are asymptomatic, but about 5% will present with hepatosplenomegaly,
periventricular brain calcification, petechial hemorrhages, and hydrops. Sensorineural deafness is a common
manifestation in affected infants. Primary infection of the mother is strongly associated with more severe
symptoms in affected infants. The mortality rate is 30% and most of the survivors will have neurologic
impairment. CMV infections in children and adults are often asymptomatic; however, the virus is responsible for 10% of infectious mononucleosis cases (characterized by an absence of sheep RBC agglutinins). CMV also
causes serious disease in immunosuppressed patients, including bone marrow recipients and AIDS patients.
Ganciclovir is the drug of choice, although foscarnet has also been used with some success.
Infections with hepatitis viruses (choices B, C, and D), if symptomatic, are characterized by jaundice, lethargy,
failure to thrive, abdominal distention, clay- colored stools, and elevated transaminase levels. Hepatitis B(choice
B) is the only viral hepatitis agent that is recognized as an important cause of neonatal disease. The infection is
usually acquired during the birth process; therapy includes hyperimmune human anti-hepatitis B serum followed
a few weeks later with active immunization with rHBsAg injections.
Herpes simplex (choice E) can also infect the neonate. The hallmark of infection is painful skin vesicles (present
in 50% of affected infants); other manifestations include encephalitis, pneumonia, hepatitis, and disseminated
intravascular coagulopathy. Massive hepatic and adrenal necrosis is common. Therapy is with acyclovir.

The correct answer is B.Streptococcus bovis is a Group D streptococcus. There is a significant association
between S. bovis bacteremia and endocarditis with carcinoma of the colon and other colonic diseases. Every
patient with S. bovis bacteremia should undergo gastrointestinal and cardiac evaluation. Up to 50% of patients
with S. bovis bacteremia are reported to have underlying colonic malignancies. In another study, 25-50% of
cases of S. bovis bacteremia were associated with endocarditis, especially in patients with preexisting valvular
lesions.
Streptococcus agalactiae(choice A) is an important cause of maternal and neonatal bacteremia and neonatal
meningitis. It is part of the normal flora of the gastrointestinal tract and the female genital tract.
Streptococcus pneumoniae(choice C) is a leading cause of community-acquired pneumonia, meningitis in
adults, otitis media (especially in children), and sinusitis. Spontaneous peritonitis due to S. pneumoniae is
reported in children with ascites from nephrotic syndrome. Asplenia predisposes patients to severe infections
with S. pneumoniae and other encapsulated organisms. S. pneumoniae infections are also more frequent and
unusually severe in patients with sickle cell anemia, multiple myeloma, alcoholism, and hypogammaglobulinemia.
S. pneumoniae is now the leading cause of invasive bacterial respiratory disease in patients with AIDS.
Streptococcus pyogenes(choice D) is the most common cause of bacterial pharyngitis. Complications include
paratonsillar abscesses, otitis media, and sinusitis. Long-term sequelae include rheumatic fever and
poststreptococcal glomerulonephritis. S. pyogenes is also responsible for many skin and soft tissue infections.
The organism also produces many toxins that produce a variety of diseases.
Streptococcus viridans(choice E), or the viridans Streptococci, are the most common cause of subacute
bacterial endocarditis, which should be suspected in cases of viridans streptococcal bacteremia. One species of
viridans Streptococci, Streptococcus milleri, is frequently associated with pyogenic abscesses, especially of the
liver.

The correct answer is B. The most probable etiology of bacterial endocarditis involving the tricuspid valve is
illicit intravenous drug use, which can introduce skin organisms into the venous system that then attack the
tricuspid valve. Staphylococcus aureus accounts for between 60% and 90% of cases of endocarditis in
intravenous drug users.
The endocarditis associated with congenital heart disease (choice A) typically involves either damaged valves
or atrial or ventricular septal defects. The tricuspid valve is not particularly vulnerable.
Rheumatic fever (choice C) most commonly damages the mitral and aortic valves, and tricuspid damage is
usually less severe and seen only when the mitral and aortic valves are heavily involved. Consequently,
secondary bacterial endocarditis involving only the tricuspid valve in a patient with a history of rheumatic fever
would be unusual.
Rheumatoid arthritis (choice D) is not associated with bacterial endocarditis.
Systemic lupus erythematosus (choice E) can produce small, aseptic vegetations on valves, but is not
associated with bacterial endocarditis.

The correct answer is A.Bartonella henselae is the infective agent of cat scratch disease, which generally
presents as regional lymphadenopathy with or without low fevers and headaches. Bartonella is a gram-variable
pleomorphic rickettsial organism that is introduced to the skin in a cat bite or scratch. It produces a self-limited
granulomatous response in the draining lymph nodes.
Borrelia burgdorferi (choice B) is a spirochetal organism that is transmitted by a tick bite (Ixodes spp.),
producing Lyme disease. Lyme disease progresses from a skin rash to fevers, headache and pain over about
one month. It may produce lymphadenopathy, but is not associated with granuloma formation.
Chlamydia psittaci (choice C) infection occurs after contact with infected bird droppings and produces an
atypical pneumonia. The central nervous system may also be involved, but lymph nodes are spared. Chlamydia
trachomatis is the chlamydial species that typically produces suppurative nodal granulomas (lymphogranuloma
venereum).
Coxiella burnetii(choice D) infection is transmitted by inhaling dusts or drinking milk from infected mammals,
especially sheep and cows. The disease in humans, Q fever, is marked by mild nonspecific symptoms or
pneumonia, and may progress to myocarditis or hepatitis.
Rickettsia prowazekii(choice E) produces epidemic (louse-borne) typhus, which is transmitted by body lice and
produces a rash akin to Rocky Mountain spotted fever. Although the organism may reside in the lymph nodes in
dormancy, it does not elicit granuloma formation.

The correct answer is D. All of the organisms listed can cause infection in infants, but Listeria monocytogenes is
the one to cause potentially fatal granulomas (granulomatis infantiseptica) following in utero infection.
Clostridium botulinum(choice A) can cause infant botulism, characterized by failure to thrive, and progressive
muscular weakness.
Escherichia coli(choice B) can cause diarrhea, pneumonia, and meningitis in infants.
Haemophilus influenzae(choice C) can cause acute epiglottitis, meningitis, pneumonia, and otitis media in
young children.
Neisseria gonorrhoeae(choice E) can be transmitted via the birth canal, and causes ophthalmia neonatorum.

The correct answer is D. Penicillin remains the first-line drug of choice for pneumococcal pneumonia, except in
patients with penicillin allergy and in the relatively few areas in which pneumococcal strains with high-level
penicillin resistance exist. Alternative therapies include erythromycin and vancomycin.
The third-generation cephalosporin cefotaxime (choice A) is not usually used for pneumococcal pneumonia.
Chloramphenicol (choice B) is not usually used for pneumococcal pneumonia.
Erythromycin (choice C) is a good alternative therapy for pneumococcal pneumonia, but is usually used only
when a penicillin allergy is present.
Vancomycin (choice E) is not the first-line therapy, but it is a good alternative in patients allergic to penicillin or
when high-level penicillin resistance (relatively uncommon) is present.

The correct answer is D. The lesions are characteristic of molluscum contagiosum, which is a typically benign
and self-limited condition caused by a poxvirus. The disease can be transmitted either venereally or through
non-venereal contact. The other viruses listed do not cause similar skin lesions. Patients with advanced HIV
infection may develop a severe, generalized, and persistent eruption, often involving the face and upper body.
Cytomegalovirus (choice A) causes congenital infections and disseminated infections in immunosuppressed
patients.
Herpesvirus 6 (choice B)causes roseola (exanthem subitum).
Parvovirus (choice C) causes aplastic crises in patients with hemolytic anemia.
Variola (choice E) is the smallpox virus.

The correct answer is E. This is a case of Neisseria gonorrhoeae arthritis. Patients are continuously susceptible
to reinfection because of antigenic variation and phase variation of the pili.
N. gonorrhoeae is not an intracellular pathogen (choice A), although it may be found intracellularly in
neutrophils after it has been phagocytized.
Ceftriaxone (choice B) is the drug of choice for N. gonorrhoeae.
Gonococci are especially susceptible to complement-mediated lysis, not resistant to it (choice C).
N. gonorrhoeae has an insignificant capsule that does not play a major role in the pathogenesis, but its capsule
is immunogenic (compare with choice D). Streptococcus pyogenes is the best known example of a
nonimmunogenic capsule, made of hyaluronic acid, but virtually all other capsules are immunogenic

The correct answer is D.Pseudomonas aeruginosa causes malignant otitis externa, which is a severe
necrotizing infection of the external ear canal. Infection tends to spread to the mastoid bone, temporal bone,
sigmoid sinus, base of the skull, meninges, and brain. Patients at increased risk include the elderly, those with
diabetes, and the immunocompromised. Pseudomonas is also associated with many other clinical syndromes,
including infection following traumatic wounds to the feet in persons wearing sneakers or rubber-soled shoes,
since the organism can be cultured from the shoes.
Hemophilus influenzae(choice A) produces a variety of clinical syndromes. H.Influenzae is the third-most
common cause of meningitis in children aged 1 month to 18 years. It is the most common cause of acute
epiglottitis, the most common cause of purulent bacterial conjunctivitis, and the second-most common cause of
otitis media. Patients at risk include those with COPD and cystic fibrosis, alcoholics, splenectomized patients,
and young patients.
Klebsiella pneumoniae(choice B) is a gram-negative organism that produces a necrotizing pneumonia in
diabetics and alcoholics. Patients typically present with an abrupt onset of fever, shaking chills, and purulent,
foul-smelling sputum.
Mucor (choice C) is a fungal infection that is particularly severe in the diabetic or the immunocompromised
patient. In the acidotic diabetic, the fungus produces a life-threatening, invasive rhinocerebral infection. The
infection begins in the nasal passages, extends into the paranasal sinuses, and spreads through the cribiform
plate to the frontal lobes of the brain. Patients typically complain of headache, facial pain, and orbital swelling.
Streptococcus pyogenes(choice E) causes bacterial pharyngitis, otitis media, and sinusitis. It is also associated with toxin-related diseases and skin infections.

The correct choice is D. This patient is most likely suffering from cytomegalovirus (CMV) retinitis. The best drug
treatment for this infection is ganciclovir.
Acyclovir (choice A) is not effective in CMV infections. It is used more for HSV type 1 and 2 infections.
Amantadine (choice B) is used either therapeutically or prophylactically for the influenza A virus.
Flucytosine (choice C) is an antifungal agent.
Zidovudine (choice E) is a first-line drug for the treatment of AIDS. The drug by itself is ineffective against CMV retinitis.

The correct answer is C. Clues to the correct answer are the underlying disease (AIDS), which predisposes
to opportunistic infections, the typical MRI appearance of the lesion (ring-enhancing mass), and the
histopathologic features (presence of encysted bradyzoites). In short, this patient has cerebral toxoplasmosis,
which represents one of the most common opportunistic infections in AIDS. Toxoplasma gondii is a protozoon
that infects humans who ingest the oocysts from cat feces or incompletely cooked lamb or pork. Only
immunodepressed patients and fetuses are vulnerable to this infection. In the fetus, toxoplasmosis causes
extensive damage to brain parenchyma and retina. Toxoplasmosis associated with AIDS manifests with
necrotizing lesions surrounded by chronic inflammation. A ring-enhancing lesion is a mass that contains a rim of
contrast enhancement (bright signal on MRI) surrounding a dark core corresponding to central necrosis. In
AIDS, the most frequent causes of a ring-enhancing lesion are primary brain lymphoma and toxoplasmosis.
Anopheles mosquitoes (choice A) transmit malaria parasites. Cerebral malaria is caused by Plasmodium
falciparum, which is able to adhere to endothelial cells of small cerebral vessels. Vascular occlusion ensues,
resulting in numerous small infarcts.
Bird droppings (choice B) represent the vehicle of infection for two of the most common opportunistic fungal
infections affecting immunocompromised patients: cryptococcosis and histoplasmosis. Cryptococcus
neoformans causes meningoencephalitis, not intracerebral necrotic lesions. Histoplasma capsulatum rarely
affects the brain. These fungi can be visualized in tissue sections by silver stains.
Cooling systems (choice D) may harbor Legionella pneumophila, spreading the bacteria in aerosolized form. L. pneumoniae is a gram-negative bacterium that causes Legionnaire's disease, a fatal form of pneumonia that
first struck participants at a meeting of the American Legion. It has been reported in immunocompromised
patients as well.
Washbasins (choice E) frequently contain Pseudomonas aeruginosa, which has also been isolated from
respirator devices, cribs, and antiseptic-containing bottles. P. aeruginosa tends to affect patients with cystic
fibrosis, severe burns, or immune impairment. It may cause bronchopneumonia, osteomyelitis, endocarditis,
external otitis, and keratitis, but not cerebral infection.

The correct answer is A. The infectious agents listed are all important causes of congenital disease. The triad of
cutaneous hemorrhages ("blueberry muffin baby"), deafness, and periventricular CNS calcifications suggests
congenital CMV infection, the most common cause of intrauterine fetal viral infection. Other manifestations
include microcephaly and hepatosplenomegaly.
Neonatal herpes (choice B) may be congenital, but more commonly is acquired during vaginal delivery. The
infection is characterized by vesicles on the skin and mucous membranes, encephalitis, or disseminated
disease.
Congenital rubella (choice C) can cause mental retardation, heart abnormalities, blindness, encephalitis, and
motor abnormalities.
Congenital syphilis (choice D) can cause death in utero, or a variety of problems including abnormal teeth,
bones, and central nervous system.
Toxoplasmosis (choice E) can be either acquired during delivery (mild) or congenital (severe). Severe infections
can cause stillbirth, chorioretinitis, intracerebral calcifications, and hydro- or microcephaly.

The correct answer is A. Acute pyelonephritis is an infectious disease involving the kidney parenchyma and the
renal pelvis. Gram-negative bacteria, such as Escherichia coli, Proteus, Klebsiella, and Enterobacter, are the
most common causative organisms in acute pyelonephritis. Laboratory evaluation will often reveal leukocytosis
with a left shift, and urinalysis typically shows pyuria, varying degrees of hematuria, and white cell casts. Since
bacteremia is present, the patient should be hospitalized and empirically started on IV ampicillin and gentamicin.
This regimen may be need to be changed, however, once the sensitivity results are available.
Erythromycin (choice B) and tetracycline (choice E) are both bacteriostatic antibiotics and would not be
recommended in a patient with a severe infection, such as acute pyelonephritis with bacteremia.
Vancomycin (choice C) is primarily used in the treatment of severe gram-positive infections.
Phenazopyridine (choice D) is a urinary analgesic, and nitrofurantoin (choice D) is a urinary tract anti-infective.
Although nitrofurantoin is indicated for the treatment of "mild" cases of pyelonephritis, as well as cystitis, this
patient's condition is severe and should be treated with appropriate antibiotics.

The correct answer is B. This is a case of tinea pedis, or athlete's foot, caused by a variety of dermatophytic
fungi, which are easily detected on alkali mounts of scraped skin as colorless, branching hyphae with
cross-walls.
Hyphae with rosettes of conidia (choice A) describes the environmental/transmission form for Sporothrix
schenckii, the agent of rose gardener's disease, which is a subcutaneous mycosis.
Budding yeasts (choice C) describes the form found in clinical specimens from patients with sporotrichosis.
Hyphae, arthroconidia, and blastoconidia (choice D) would be found in clinical specimens from patients infected
with Trichosporon beigelii (white piedra), which is a superficial mycosis of the hair of the head.
Pigmented, septate hyphal fragments (choice E) would be found in cases of phaeohyphomycosis, a diverse
group of cyst-forming subcutaneous, pigmented (dematiaceous) fungi, rare in the U.S.
Short, curved hyphae and round yeasts (choice F), or the "spaghetti and meatball" presentation in clinical
specimens, are characteristic of Malassezia furfur, the agent of pityriasis versicolor.

The correct answer is B. The positive leukocyte esterase test indicates the presence of neutrophils in the urine,
suggesting a bacterial infection. The nitrite test exploits the fact that most Enterobacteria (gram-negative enteric
rods) are able to form nitrite from nitrate; thus, the nitrite test is used to diagnose urinary tract infections. One
limitation of this method is the fact that enterococci (gut streptococci) do not produce nitrite from nitrate, but can
nonetheless cause urinary tract infections. Enterococcal urinary tract infections are often nosocomial and
classically acquired in the intensive care unit, although they can occur in other settings.
Enterobacter sp. (choice A), Escherichia coli(choice C), Klebsiella pneumoniae(choice D), and Pseudomonas
aeruginosa(choice E) can cause urinary tract infections and would usually be picked up by the dipstick for
nitrites. False-negative results might still be seen with these organisms if the infection was light, the bladder had
been recently emptied prior to collection, and the urine was "new" and had not yet grown enough bacteria to
produce a positive result.

The correct answer is E. This patient has acute hepatitis B. The "window period" refers to that period in
infection when neither hepatitis B surface antigen (HBsAg) nor its antibody (HBsAb) can be detected in the
serum of the patient. It is an immunologically mediated phenomenon caused by the precipitation of
antigen-antibody complexes in their zone of equivalent concentrations and, thereby, their removal from the
circulation. Because of this, the first thing that will happen in the window period is that the serum will become
negative for the surface antigen (HBsAg), as that antigen is precipitated out of the serum by developing levels
of its specific antibody (HBsAb).
HBcAg is not typically measured (choice A) in the serum.
Levels of the c-core antibody HBc IgG (choice B) and HBeAg (choice C) do not have a relationship to the
window period.
Levels of HBsAb (choice D) will not be detectable until there is antibody excess, and the patient is leaving the
window period.

The correct answer is D. All of the organisms listed are protozoa. There are two intestinal protozoa specifically
associated with AIDS that can cause transient diarrhea in immunocompetent individuals but can cause
debilitating, and potentially life-threatening chronic diarrhea in AIDS patients. These organisms are Isospora
belli, treated with trimethoprim-sulfamethoxazole (or other folate antagonists) and Cryptosporidium parvum (no
treatment presently available).
Diphyllobothrium latum(choice A) is the fish tapeworm and occasionally causes diarrhea.
Entamoeba histolytica(choice B) and Giardia lamblia(choice C) are both causes of diarrhea, but they are not
specifically associated with AIDS.
Microsporidia (choice E) are a protozoan cause of diarrhea, but produce spores rather than oocysts.