USMLE Microbiology Prt 5

The correct answer is C. Herpes simplex is an enveloped, linear DNA virus that is a very common infectious
agent; most adults will have anti-Herpes simplex antibodies in their serum, although many may not have ever
had any clinical signs of disease. The hallmark of this disease is painful skin vesicles often called "cold" sores,
or "fever" blisters to denote the precipitating event that preceded the appearance of the lesions. The virus has
a propensity to become latent in the host, finding safe refuge in nervous tissue. Activation of the infection
occurs following mild trauma (e.g., a visit to the dentist), hormonal changes (e.g., menses), and
immunosuppression (e.g., following organ transplantation). Other, more serious, manifestations of disease
include encephalitis, pneumonia, and hepatitis; these are particularly likely to be seen in immunodeficient
patients such as those with AIDS.
Adenoviruses (choice A) are naked, linear, double-stranded DNA viruses that cause acute, usually self-limiting,
influenza-like illnesses occurring in the fall and winter. The symptoms include pharyngitis, fever, cough, and
general malaise. Epidemic pharyngoconjunctivitis and pneumonia can occur in closed populations such as
military installations.
Coxsackie viruses (choice B) are naked, single-stranded, polycistronic viruses with an RNA genome. They are
divided into groups A and B based on their virulence in suckling mice. Coxsackie A group causes generalized
myositis and flaccid paralysis, which is rapidly fatal to infant mice, whereas group B produces less severe
lesions of the heart, pancreas, and central nervous system. In man, Coxsackie A causes herpangina and
hand-foot-and-mouth disease, while Coxsackie B is seen in patients with pleurodynia, myocarditis, and
pericarditis. Both groups cause upper respiratory infections, febrile rashes, and meningitis.
Herpes zoster (choice D), the varicella virus, is an enveloped, double-stranded DNA virus that is a very common
infectious agent in children. Chickenpox is a mild, self-limiting illness in children that is evidenced as a fever
followed by a macular rash that progresses to papules, then vesicles of the skin and mucous membranes.
Shingles is a recurrence of a previously latent varicella infection in which the virus has taken refuge in sensory
ganglia of spinal or cranial nerves. Various factors that decrease the immune status of the patient contribute to
the exacerbation of the infection. Severe dermatomal pain occurs with a vesicular eruption, fever, and malaise.
Papilloma viruses (choice E) are members of the Papovavirus family. They are non-enveloped and possess a
double-stranded, circular DNA genome. They cause skin, plantar, and genital warts; some serotypes of human
papilloma viruses (e.g., HPV-16) are associated with penile, laryngeal, and cervical carcinomas.

The correct answer is B. In a sense, cervical carcinoma can be considered a "sexually transmitted disease,"
since human papilloma virus (typically strains HPV-16 or HPV- 18) is transmitted venereally. In our culture,
HPV-related dysplasia of the cervix is common, even in the 18-28 year old group, and carcinomas of the cervix
(more rare now than in the past due to screening and aggressive therapy of dysplasia) can occur in the 35-45
age group. The other cancers listed typically occur in older age groups and are not as clearly related to
venereal agents.

The correct answer is C. Herpes simplex is an enveloped, linear DNA virus that is a very common infectious
agent; most adults will have anti-Herpes simplex antibodies in their serum, although many may not have ever
had any clinical signs of disease. The hallmark of this disease is painful skin vesicles often called "cold" sores,
or "fever" blisters to denote the precipitating event that preceded the appearance of the lesions. The virus has
a propensity to become latent in the host, finding safe refuge in nervous tissue. Activation of the infection
occurs following mild trauma (e.g., a visit to the dentist), hormonal changes (e.g., menses), and
immunosuppression (e.g., following organ transplantation). Other, more serious, manifestations of disease
include encephalitis, pneumonia, and hepatitis; these are particularly likely to be seen in immunodeficient
patients such as those with AIDS.
Adenoviruses (choice A) are naked, linear, double-stranded DNA viruses that cause acute, usually self-limiting,
influenza-like illnesses occurring in the fall and winter. The symptoms include pharyngitis, fever, cough, and
general malaise. Epidemic pharyngoconjunctivitis and pneumonia can occur in closed populations such as
military installations.
Coxsackie viruses (choice B) are naked, single-stranded, polycistronic viruses with an RNA genome. They are
divided into groups A and B based on their virulence in suckling mice. Coxsackie A group causes generalized
myositis and flaccid paralysis, which is rapidly fatal to infant mice, whereas group B produces less severe
lesions of the heart, pancreas, and central nervous system. In man, Coxsackie A causes herpangina and
hand-foot-and-mouth disease, while Coxsackie B is seen in patients with pleurodynia, myocarditis, and
pericarditis. Both groups cause upper respiratory infections, febrile rashes, and meningitis.
Herpes zoster (choice D), the varicella virus, is an enveloped, double-stranded DNA virus that is a very common
infectious agent in children. Chickenpox is a mild, self-limiting illness in children that is evidenced as a fever
followed by a macular rash that progresses to papules, then vesicles of the skin and mucous membranes.
Shingles is a recurrence of a previously latent varicella infection in which the virus has taken refuge in sensory
ganglia of spinal or cranial nerves. Various factors that decrease the immune status of the patient contribute to
the exacerbation of the infection. Severe dermatomal pain occurs with a vesicular eruption, fever, and malaise.
Papilloma viruses (choice E) are members of the Papovavirus family. They are non-enveloped and possess a
double-stranded, circular DNA genome. They cause skin, plantar, and genital warts; some serotypes of human
papilloma viruses (e.g., HPV-16) are associated with penile, laryngeal, and cervical carcinomas.

The correct answer is B. The tumor is hepatocellular carcinoma, which usually develops in the setting of cirrhosis due to a variety of damaging agents, including hepatitis B virus (HBV) infection, alcohol use, and hemochromatosis.

EBV (choice A) is associated with Burkitt's lymphoma and nasopharyngeal carcinoma.

HHV 8 (a member of the herpes family, choice C) is associated with Kaposi's sarcoma.

HPV (human papillomavirus, choice D) is associated with cervical, penile, and anal carcinoma.

HTLV-1 (human T-lymphocyte virus, choice E) is associated with adult T-cell leukemia.

The correct answer is D. The gross photograph shows a stomach with a perforated peptic ulcer. Perforation of a
peptic ulcer is potentially fatal, because of either peritonitis with sepsis or sudden exsanguination (if the perforation
damages one of the many arteries of the stomach). Peptic ulcer disease, gastritis, and possibly gastric carcinoma
and gastric lymphoma have been strongly associated with Helicobacter pylori colonization of the mucus layer
covering the gastric mucosa. Colonization is associated with destruction of the mucus layer, thereby destroying its
protective function.
Cryptosporidium parvum(choice A) causes diarrhea that is severe in immunocompromised patients.
Entamoeba histolytica(choice B) produces dysentery-like symptoms or can cause liver abscess.
Escherichia coli(choice C) causes a variety of diarrheal diseases and can infect the bladder and soft tissues.
Mycobacterium tuberculosis(choice E) causes tuberculosis, characterized by granuloma formation, especially in the
lungs.

indicative of a defect in cell-mediated immunity. It is caused by either Bartonella henselae or Bartonella
quintana. The domestic cat is the reservoir for these organisms and they are usually transmitted to humans via
a cat scratch or cat bite. Patients with this illness usually have multiple skin lesions and extracutaneous
manifestations involving liver and bone. Diagnosis is usually based on characteristic histopathologic findings
including plump "epithelioid" endothelial cells and mitotic figures. A macrolide, such as erythromycin or
azithromycin, is the drug of choice for the infection.
Human papillomavirus (choice B) causes warts. Infection can present as a sessile wart or as condyloma acuminatum, which are fleshy soft growths that coalesce into large masses. When cellular immunity is
depressed, as in AIDS, the condylomata acuminatum proliferate.
Molluscum contagiosum virus (choice C) is a pox virus that is spread by close person-to-person contact.
Infection produces a firm nodule that often becomes umbilicated, and may resolve by discharging its contents.
In AIDS, the lesions do not resolve, but enlarge and spread.
Rickettsia prowazekii(choice D) is the cause of epidemic typhus. It is spread by the human body louse,
Pediculus humanis. Its reservoirs are humans and flying squirrels.
Treponema pallidum(choice E) is the spirochete that causes syphilis. The characteristic primary lesion is a
chancre (a painless, indurated ulcer) at the site of inoculation.

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.
Neisseria gonorrhoeae(choice C) causes gonorrhea, which usually does not involve the CNS.
Herpes simplex I (choice B) can cause an encephalitis that typically involves the frontal and temporal lobes.

The correct answer is C. The only spirochete among the choices is Leptospira interrogans, so even if you didn't know the diseases these organisms produce, you may have been able to answer the question.
Leptospirosis, which this patient has, is caused by a spirochete; if you were presented with a list of spirochetes in the choices, the phrase "Shepherd's crook" should tip you off to Leptospira. Clinically, leptospirosis may range from nearly asymptomatic, or at least indistinguishable from other minor flu-like illnesses, to a potentially fatal form (Wal's disease) with jaundice, bleeding, renal failure, and skeletal muscle necrosis. Spread is via contact with blood or urine from infected animals, notably rats. Leptospirosis is found worldwide, but its more severe forms are most likely to occur in the tropics.

Brucella abortus (choice A) is a gram-negative coccus and is one of the causes of brucellosis.

Brucella melitensis(choice B) is a gram-negative coccus and is one of the causes of brucellosis.

Pseudomonas mallei (choice D) and Pseudomonas pseudomallei(choice E) are small gram-negative bacilli that cause melioidosis.

The correct answer is E. The presence of hepatitis B surface antigen (HBsAg) along with hepatitis B core IgM
antibody (HBc IgM Ab), and the absence of hepatitis B surface antibody (HBsAb) indicates the presence of the
early stages of an acute infection with Hepatitis B. The presence of antibody to Hepatitis C (HCV) only indicates
exposure, but not a specific time of exposure; however, 85% of patients who are infected with HCV develop
chronic infections, indicating that this patient has an 85% chance of having a dual infection with HBV and HCV.
The acute or chronic HCV infection can be confirmed by PCR.
A dual infection of HBV and HAV (choice A) is not plausible since the IgM anti-HAV serology is negative.
Hepatitis A does not cause chronic disease (choice B).
Chronic HBV infection (choice C) is unlikely because the patient has HBc IgM Ab, which is characteristic of an
acute infection, rather than a chronic infection.
Hepatitis C infection (choice D) is not confirmed by these data because the presence of HCV Ab only indicates
exposure to the virus, and not the state of infection. This could be caused by exposure at some earlier time
(the elevated serum transaminases might be due to HBV infection). An active or chronic HCV infection can only
be confirmed by PCR.

The correct answer is E. The Neisseria are gram-negative diplococci, and have two clinically important species,
N. meningitidis and N. gonorrhoeae.N. meningitidis can either cause a sudden, fulminant, life-threatening meningitis or meningococcemia with a vasculitic purpura and disseminated intravascular coagulation. The latter
presentation may be complicated by adrenal involvement, precipitating the usually fatal
Waterhouse-Friderichsen syndrome with coagulopathy, hypotension, adrenal cortical necrosis, and sepsis. N.
gonorrhoeae causes gonorrhea and ophthalmia neonatorum (choice D), a neonatal eye infection.
Classical dysentery (multiple, small-volume stools with blood, mucus, and pus associated with abdominal cramps
and tenesmus); (choice A) is caused by Shigella dysenteriae. A similar syndrome can be caused by other
bacteria or amoebae as well.
Erythema chronicum migrans (choice B) is the pathognomonic dermatologic manifestation of Lyme disease. This
skin lesion consists of an expanding erythematous lesion, with central clearing.
Bacterial myocarditis (choice C) is uncommon, and when it does occur, it usually involves Staphylococcus aureus
or Corynebacterium diphtheriae.

The correct answer is A. The rare disease illustrated is subacute sclerosing panencephalitis (SSPE), which
66
typically presents as in the question stem. SSPE appears to be due to a combination of persistent, possibly
abnormal measles virus and to autoimmune damage caused by antibodies directed against the virus. Many
patients developing SSPE have had measles at 2 years of age or younger; there is typically a six-year interval
between measles infection and symptom development. A small proportion of cases have followed vaccination
with live measles virus. Unfortunately, no effective therapy has been developed, although some experimental
work with drugs such as isoprinosine shows some promise.
Post-infectious encephalomyelitis, rather than SSPE, can follow mumps (choice B) or varicella (choice E). Latent
infection with the varicella virus (choice E) causes shingles (herpes zoster).
Papilloma viruses (choice C) are associated with warts. They do not usually infect the brain.
Poliovirus (choice D) causes gastrointestinal disturbances, viremia, and paralysis

The correct answer is B. Aztreonam is a monobactam. Since the basic ring structure is different from penicillins,
there is no cross-allergenicity, and it can be safely given to those who have had severe reactions to penicillins.
Aztreonam is highly active against gram-negative bacteria, but has no activity against gram-positive bacteria or
obligate anaerobes. It is not β-lactamase resistant.
68
In general, there is cross-allergenicity between all the penicillins, since most people react to breakdown
products of the β-lactam ring structure common to all penicillins. Patients who have experienced an allergic
reaction to a penicillin may also be sensitive to cephalosporins.
Ampicillin (choice A) is a third-generation penicillin. It is a broad-spectrum penicillin that is active against certain
gram-negative and gram-positive bacteria. It is also one of the main antibiotics (along with clindamycin) that can
lead to antibiotic-induced pseudomembranous colitis.
Cefoperazone (choice C) is a third-generation cephalosporin. It is a broad-spectrum cephalosporin that is active
against many gram-negative and gram-positive bacteria. It contains a methylthiotetrazole side chain that can
cause a vitamin K deficiency and disulfiram-like reaction to alcohol. Many people also experience diarrhea as a
side effect. It is a drug of choice in people with impaired renal function because 60% is eliminated by the biliary
route.
Cephalexin (choice D) is a first-generation cephalosporin. It is mainly active against gram-positive bacteria.
Methicillin (choice E) is a second-generation penicillin. It is active against many gram-positive organisms. When
given in high doses for more than 2 weeks, it can cause interstitial nephritis, hepatitis, and neutropenia.
Oxacillin (choice F) is a second-generation penicillin. It is used against many gram-positive organisms. When
used in high dose for more than 2 weeks, it can cause hepatitis and neutropenia.
Ticarcillin (choice G) is a fourth-generation penicillin. Fourth-generation penicillins are extended-spectrum
agents that are active against many gram-positive and gram-negative bacteria, including Pseudomonas and
many Enterobacteriaceae. Ticarcillin is given with clavulanic acid, a β-lactamase inhibitor. Since ticarcillin
is a disodium salt, it causes a large salt load, which can lead to salt retention and hypokalemia.

The correct answer is A. The answer to this question requires that the student realize that pus consists of
bacteria and dead and dying neutrophils. This fact, taken along with the highly characteristic case history,
reflects that the patient has a typical pneumonia. In the United States, the most common agent of this would be
Streptococcus pneumoniae, a gram-positive extracellular pathogen rich in teichoic acids and peptidoglycan,
which elicit the neutrophilic exudate.
The capsule of this organism is a polysaccharide and primarily elicits an antibody response rather than
attracting neutrophils (choice B).
Streptococcus pneumoniae is an extracellular, not an intracellular (choice C) organism.
Streptococcus pneumoniae is alpha hemolytic, not beta hemolytic (choice D).
Streptococcus pneumoniae does produce an IgA protease (choice E) that enhances the ability of the organism
to infect the respiratory mucosa, but this does not contribute to pus formation.

The correct answer is B. Follicular dendritic cells in the germinal centers of lymph nodes are important
reservoirs of HIV. Although some follicular dendritic cells are infected with HIV, most viral particles are found on
the surface of their dendritic processes. Follicular dendritic cells have receptors to the Fc portion of
immunoglobulins that serve to trap HIV virions coated with anti-HIV antibodies. These coated HIV particles retain
the ability to infect CD4+ T cells as they traverse the dendritic cells.
B lymphocytes (choice A) have a surface marker (CD21 protein-a complement receptor) to which an
Epstein-Barr envelope glycoprotein can bind. The virus associates with the host cell genome, producing a latent
infection. These B cells undergo polyclonal activation and proliferation.
Ganglion cells (choice C), particularly the satellite cells around the ganglion cells in the dorsal root ganglia, can
be infected by varicella-zoster. Herpes type I and II infect neurons that innervate skin and mucous membranes.
Oligodendrocytes (choice D) are directly infected by two viruses, JC virus (a polyomavirus) and measles virus.
JC virus causes progressive multifocal leukoencephalopathy (PML), and measles virus produces a latent
syndrome called subacute sclerosing panencephalitis (SSPE).
Peripheral nerves (choice E) are indirectly affected by HIV virus in the AIDS-associated myopathy. The disease
is characterized by a subacute onset of proximal muscle weakness, sometimes with pain, and elevated levels of
creatine kinase. The muscles and nerves are infiltrated with mononuclear cells, including HIV-positive
macrophages.

The correct answer is E. Hepatitis E is an important, and until recently, unrecognized cause of epidemics of
enterically transmitted acute hepatitis. Hepatitis E is caused by an enterically transmitted virus that occurs
primarily in India, Asia, Africa, and Central America. Infection with the virus is associated with a very high
mortality among pregnant women.
CMV (choice A) can cause acute hepatitis, but the disease is usually mild and often goes unrecognized, except
in profoundly immunosuppressed patients.
Hepatitis A virus (choice B) is the major cause of epidemics of enterically transmitted viral hepatitis, but is not a
significant cause of mortality in pregnant women.
Hepatitis C virus (choice C) is usually transmitted parenterally, rather than enterically, and is not a significant
cause of mortality in pregnant women.
Herpes simplex (choice D) usually causes significant hepatitis only in profoundly immunosuppressed patients.

The correct answer is B. Any time you see the development of diarrhea in the same question stem as the words "treated with antibiotics," you should immediately think of pseudomembranous colitis. Pseudomembranous colitis is caused by Clostridium difficile and typically occurs as a result of treatment with clindamycin or ampicillin. You would confirm your suspicion by sending a stool culture to be tested for the presence of the C. difficile toxin.

Chloramphenicol's (choice A) most test-worthy side effect is aplastic anemia, not diarrhea. In addition, you might have been able to eliminate this choice simply because of the extremely low probability that this patient would receive this antibiotic in the USA.

Gentamicin's (choice C) key side effects include ototoxicity and nephrotoxicity.

Metronidazole (choice D) and vancomycin (choice E) do not cause pseudomembranous colitis; they are used to treat it.

The correct answer is B. The boy is displaying the characteristic symptoms of cystic fibrosis with bronchiectasis
(accounting for the foul-smelling sputum) and pancreatic insufficiency, producing steatorrhea. However, the
question directly relates to the infectious agent causing the pneumonia, Pseudomonas aeruginosa, an
oxidase-positive bacteria with a fruity aroma. P. aeruginosa is a strictly aerobic gram-negative rod that produces
an exotoxin (exotoxin A) that ADP-ribosylates, and therefore inhibits, eukaryotic elongation factor 2 (eEF-2),
which is a G-protein involved in the eukaryotic translation of proteins.
The cystic fibrosis transmembrane conduction regulator (CFTR) protein (choice A) is the product of the cystic
fibrosis gene, a large 24 exon gene located on the long arm of chromosome 7 (7q31). This protein has two
nucleotide binding domains. In 70% of the families with the disease, a three base pair deletion at codon 508 of
exon 10 results in the deletion of a phenylalanine (DF508). This codon is critical for one of the nucleotide binding sites of the CF gene product, resulting in poor function of a phosphorylation-regulated chloride ion
channel in the apical membrane of epithelial cells. The defective chloride channel leads to impaired secretion of
chloride in the lumen and promotes the absorption of sodium inside the cells. Water follows, concentrating the
airway secretions. The increased viscosity of the airway secretions predisposes the patient to repeated
infections.
A GTP-binding protein similar to the one coupled with α2-adrenergic receptors (choice C) refers to a
G-protein that inhibits adenylate cyclase, lowering cAMP. This G-protein is sensitive to Pertussis toxin.
A GTP-binding protein similar to the one coupled with β-adrenergic receptors (choice D) refers to a
G-protein that stimulates adenylate cyclase, increasing cAMP. This G-protein is sensitive to cholera toxin.
The product of the cystic fibrosis gene is a phosphorylation-regulated chloride channel in the apical membrane
of epithelial cells (choice E).

The correct answer is C. Entamoeba are relatively common enteric pathogens that can produce asymptomatic
infection or more severe disease characterized by mucosal ulcerations and submucosal spread causing
abdominal distress and liquid stools. Stools may show either trophozoite forms or the typical spherical cysts.
Several species of Entamoeba are seen, including Entamoeba coli and E. hartmanni. E. histolytica cysts
characteristically are spherical in shape, 10-20 μm in diameter, and have granular cytoplasm containing 1, 2, or
4 nuclei.
Cryptosporidium parvum(choice A) infections occur in the immunocompromised population and may cause
severe diarrhea. The organism presents as minute (2-5 μm) intracellular spheres or arc-shaped merozoites
under normal mucosa, and can be difficult to appreciate by light microscopy. Cysts in the stool are too small
(4-5 μm) to be confused with Entamoeba.
Dientamoeba fragilis(choice B) is an intestinal amoeba that also may produce an infectious diarrhea. It does not
have a cyst form, and only the trophozoite forms are seen in stools.
Giardia lamblia(choice D) is a flagellate protozoan that infects the stomach and small intestine when
contaminated water is ingested. Diagnosis is usually made by examining duodenal contents; however, the stools
may contain the oval or elliptical cysts, which are thick-walled and measure 8-14 μm in diameter. Spherical cysts
are not seen in Giardia infection.
Isospora belli(choice E) produces self-limited intestinal infections mostly in the tropics, where fever and diarrhea
may last weeks to months. The stool-borne cysts are much larger than Entamoeba (30 x 15 μm), are
asymetrical, and are typically almond-shaped.

The correct answer is D. Latin America and "pilot's wheel" budding yeast are clues for paracoccidioidomycosis.
This disease is caused by Paracoccidioides brasiliensis, a dimorphic fungus that is found as a multiply-budding yeast in tissues.

Clues for questions about blastomycosis (choice A) would include spending time in states east of the Mississippi River and fairly large yeast.

Clues for questions about coccidioidomycosis (choice B) would be spending time in the southwestern deserts of the United States and spherules filled with endospores.

Clues for questions about histoplasmosis (choice C) would be spending time in the Ohio, Mississippi, and Missouri River Valleys and finding tiny yeast forms in macrophages.

Clues for questions about sporotrichosis (choice E) would be skin lesions in rose gardeners.

The correct answer is B. Gray "baby" syndrome is a disorder that occurs in newborns who have either received
chloramphenicol immediately after birth or whose mothers have received the medication close to the delivery
date. Symptoms typically appear in the following order: abdominal distention with or without emesis, progressive
pallid cyanosis, and vasomotor collapse, frequently accompanied by irregular respiration. Death can occur as
early as a few hours after onset of signs and symptoms. Other symptoms may include: loose, greenish stools, a
refusal to suck, ashen color (implied by the name gray baby syndrome), and lactic acidosis. Chloramphenicol is
an antimicrobial agent used in the treatment of serious infections when less toxic alternatives are inappropriate.
Aztreonam (choice A) is a beta-lactam antibiotic used primarily in the treatment of gram-negative infections of the
urinary tract, lower respiratory tract, and skin, and for intra-abdominal infections. The use of this agent in
pregnant or nursing women and infants is considered to be safe and effective.
Clindamycin (choice C) is an anti-infective agent used in the treatment of serious infections when less toxic
alternatives are inappropriate. Although the agent is considered to be safe and effective during pregnancy, it is
associated with the development of pseudomembranous colitis and agranulocytosis.
Metronidazole (choice D) is an antibiotic used primarily in the treatment of anaerobic infections. The use of
metronidazole should be restricted in pregnancy since newborns have a decreased ability to metabolize this
medication. When the elimination of metronidazole is decreased, the severity of adverse reactions increases.
Adverse reactions include peripheral neuropathy, seizures, irritability, and profound gastrointestinal
disturbances.
Sulfamethoxazole/trimethoprim (choice E) is associated with the development of kernicterus, which is a disorder
that can cause abnormal cerebral development in infants. The majority of infants with this disorder generally die
within a few weeks of birth. Those infants who survive are often mentally retarded, deaf, or physically impaired.

The correct answer is E. Hepatitis B infection can produce a wide variety of clinical outcomes. The most
common outcome (60% to 65%), however, turns out to be subclinical disease followed by complete recovery.
The other choices listed show other possible outcomes, and their statistical impact is considered with the
discussions of individual choices.
Approximately 20% to 25% of infected persons develop acute hepatitis (choice A), which is followed in 99% of
these cases by recovery and in about 1% of cases by fulminant hepatitis.
Approximately 5% to 10% of cases become "healthy" carriers (choice B).
Approximately 4% of cases develop persistent infection, 67% to 90% of which then recover (choice C) and 10%
to 33% of which have chronic hepatitis (choice D).

The correct answer is E. The clinical manifestations (fever, headache, nuchal rigidity, and low Glasgow coma
score), along with the CSF findings (increased neutrophils, elevated protein, and reduced glucose), strongly
indicate acute pyogenic (bacterial) meningitis as the underlying condition. Of the microorganisms listed, either
Neisseria meningitidis or Streptococcus pneumoniae can cause this form of meningitis; however, Streptococcus
pneumoniae is by far the most frequent organism causing acute meningitis in elderly patients.
Arboviruses and herpesviruses (choices A and B) can cause an encephalitis characterized by lymphocytic
infiltration of the brain parenchyma and leptomeninges. In this case, CSF findings would include an increased
number of lymphocytes and a normal glucose concentration.
Mycobacterium tuberculosis(choice C) may cause a chronic meningoencephalitis, with a prolonged clinical
course. It is characterized pathologically by a dense granulomatous infiltrate of the base of the brain.
Associated CSF findings include increased lymphocytes and normal or slightly decreased glucose.
Neisseria meningitidis(choice D) is the classic etiologic agent associated with acute pyogenic meningitis, but it
usually affects adolescents and young adults. In neonates, the most frequent organisms include Escherichia
coli and group B streptococci; in infants and children, Hemophilus influenzae; and in the elderly, Streptococcus
pneumoniae and Listeria monocytogenes.

The correct answer is B. The patient has primary atypical pneumonia caused by Mycoplasma pneumoniae.
These organisms are fastidious and difficult to culture in the laboratory, however serodiagnosis can be most
helpful. Patients produce one or two heterophile antibodies during the course of the infection: one agglutinates
human O+ RBCs in the cold (the cold hemagglutinins) while the other causes the agglutination of a strain of
Streptococcus salivarius termed strain MG (the Strep MG agglutinins). Mycoplasma are susceptible to the
macrolide family of antibiotics (erythromycin, clarithromycin, etc.). These organisms lack a cell wall, hence they
are indifferent to antibiotics that interfere with peptidoglycan synthesis, such as penicillin (choice D) and
ampicillin (choice A).
Oxygen and external cooling (choice C) are therapeutic measures that are used in the treatment of severe
respiratory diseases such as pneumococcal pneumonia.
Ribavirin (choice E) is used in the treatment of respiratory syncytial virus infection in infants. This is the most
common cause of hospitalization for respiratory disease in the very young, probably because aerosol
administration of the antiviral compound is best accomplished in a hospital setting.

The correct answer is B. The initial history given is classic for measles, with the appearance of Koplik's spots (white spots on the buccal mucosa) followed by a rash beginning along the neck and hairline and spreading to the trunk and extremities. The sequela this child is experiencing is post-infectious encephalomyelitis, which can follow either infection with measles, varicella, rubella, mumps, or influenza, or vaccination with vaccinia vaccine or rabies vaccine derived from nervous tissue. Treatment is supportive, with a mortality of 15 to 40%; survivors frequently have significant permanent neurologic deficits. The pathologic finding is perivenous microglial involvement with demyelination.

Choice A describes the findings of progressive multifocal leukoencephalopathy, a demyelinating disease caused by infection with JC virus, especially in immunocompromised individuals.

Choice C describes the findings of poliomyelitis, a paralytic disease affecting the ventral horn of the spinal cord and motor cortex, caused by an enterovirus (poliovirus).

Choice D describes the findings in herpes encephalitis, which typically affects the inferomedial temporal lobes and orbitofrontal gyri.

Choice E describes the findings in tuberculous meningitis, caused by M. tuberculosis.

The correct answer is E. This patient has Pneumocystis carinii pneumonia, the most common atypical
pneumonia in AIDS patients. The next most common pool of at-risk individuals consists of premature infants.
Bone marrow transplant patients (choice A) would be immunologically compromised, and might be a second
at-risk group (behind premature infants), but would not be the first choice on this list.
Hospitalized adults on antibiotic therapy (choice B) might be slightly immunologically compromised or stressed,
but are not the major patient pool at risk for this infection in the U.S.
Late-term pregnant women (choice C) are not immunologically compromised and would not be at risk.
Normal adults in periods of stress (choice D) are not particularly susceptible to opportunistic pathogens.
Preschool children (choice F) have normal rates of exposure to P. carinii, as would any adult, but are not
susceptible to serious pneumonia with this agent unless they are immunologically suppressed.
Sexually active adolescents (choice G) would have the same infection rates as normal infants and adults, but
again would not be susceptible to life-threatening pneumonia. P. carinii is transmitted via aerosols, not sexual
activity.
Third-trimester fetuses (choice H) are not susceptible to infection with this fungus, although they may become
infected in utero with HIV.