USMLE Microbiology Prt 8

Explanation
The correct answer is C. The disease is tularemia, which may be spread by handling rabbits or
rabbit skins, or by bites from ticks that feed on the blood of wild rabbits. The causative
organism is Francisella tularensis, a gram-negative coccobacillus. The disease begins as a
rupturing pustule followed by an ulcer, with involvement of regional lymph nodes. More serious
cases can be complicated by bacteremia, splenomegaly, rash, pneumonia, or endotoxemic shock.
Borrelia(choice A) spp. cause relapsing fever (transmitted by ticks and lice) and Lyme disease
(transmitted by ticks).
Brucella(choice B) causes brucellosis after ingestion of contaminated milk or contact with
infected livestock.
Leptospira (choice D) causes leptospirosis and Weil's disease; the organism is acquired by
ingestion of water contaminated with animal urine.
Listeria (choice E) causes listeriosis after contact with contaminated milk, vegetables, or
with transplacental transmission.

Explanation
The correct answer is A.Candida albicans produces oral thrush, an AIDS-defining lesion, which
is common in acute HIV disease, and becomes increasing common as the CD4 + cell count falls.
The lesions are usually painless. Diagnosis is by demonstration of pseudohyphae using a wet
smear with confirmation by culture.
Although cytomegalovirus (choice B) is associated with numerous clinical scenarios in the AIDS
population, including odynophagia (painful swallowing), it would not produce white plaques on
the oral mucosa.
Herpes Simplex I (choice C) produces vesicular lesions occurring in clusters in the oral
cavity. There is an increased risk of herpes infections in the AIDS group, but the lesions do
not resemble those described in the question.
Human herpesvirus 8 (choice D) is the causative agent of Kaposi's sarcoma, a malignancy arising
from endothelial cells that appears as hemorrhagic nodules in different organ systems. It is
the most common cancer in the HIV infected population.
Human papilloma virus (choice E) is associated with a variety of lesions, including warts and
intraepithelial neoplasias of the vulva and cervix. It is associated with anal condyloma, which
can occur in the AIDS population.

Explanation
The correct answer is E. IgM immunoglobulin directed against Toxoplasma would provide evidence of infection in the newborn baby. IgM is the only antibody that a baby can form with an acute
infection. It is the first antibody that is formed in an infection and it is also the antibody
that is present on the surface of immature and mature B cells.
IgA (choice A) is the antibody that is produced in response to mucosal infections. It cannot
cross the placenta. The baby's immune system is still not well developed at birth and cannot
produce IgA immunoglobulin. In adults or older children, IgA is exists in serum in a monomeric
form but is present in seromucous secretions as a dimer.
IgE (choice B) is the antibody that is produced in response to an allergen. It cannot cross the
placenta, and is not present in the baby at birth or for some time after birth. The baby's
immune system is still not well developed at birth and can initially only form IgM
immunoglobulin.
IgG 1 (choice C) and IgG4 (choice D) immunoglobulins might be present in the baby, but would
not indicate infection in the infant. The presence of these isotypes would indicate that the
mother was infected with the organism and produced antibody that was transported across the
placenta in utero. IgG is the only immunoglobulin that can cross the placenta, providing
protection to the baby during the first few months of life.

Explanation
The correct answer is B. Transformation is the uptake and integration of naked DNA by a
bacteria from the environment. Transformation can be induced in the laboratory (a technique
used to introduce gene-carrying plasmids into bacteria) or, relatively uncommonly, occurs
naturally. Only a few medically important species undergo natural transformation: Haemophilus
species, Streptococcus species, Neisseria gonorrhoeae, and Helicobacter pylori.

Explanation
The correct answer is D. This question requires you to appreciate the key structural difference
between gram-positive and gram-negative microorganisms. Most gram-negatives (other than
exceptional microorganisms, such as Mycoplasma, that lack a cell wall) have a more complex cell
envelope than gram-positive microorganisms. It includes both a cytoplasmic membrane as well as
an outer membrane. Between these two membranes is the periplasmic space, which contains enzymes
such as phosphatase and penicillinase, binding proteins for the transport of various nutrients,
and peptidoglycan, as well as a portion of the lipoprotein that firmly anchors the outer membrane to the peptidoglycan. Gram-positives do not have outer membranes.
Choice A, the cell envelope, is incorrect because both gram-positive and gram-negative
microorganisms have this structure, which is defined as all the layers that enclose the cytosol
of the bacterium. It is the composition of the envelope that differs between gram-positives and
gram-negatives.
Choice B, exotoxin, is not exclusive to gram-negative microorganisms, but is also found in some
gram-positives. By contrast, endotoxin (lipopolysaccharide; LPS) is found exclusively in gramnegatives.
Choice C, peptidoglycan, is found in the cell walls of both gram-positive and gram-negative
microorganisms. Note that there is a larger amount of peptidoglycan in gram-positive
microorganisms.
Choice E, teichoic acids, are found exclusively in gram-positive organisms.

Explanation
The correct answer is D. This patient has toxic shock syndrome (TSS), a multisystem syndrome
caused by a toxin (TSST-1) formed by certain strains of S. aureus. TSS usually affects several
organ systems (gastrointestinal, renal, hepatic, hematopoietic, musculoskeletal, pulmonary) and
can result in death. TSS has historically been associated with the use of tampons in young
women, but can also occur in other patient populations. Fever, hypotension, diarrhea, and
diffuse rash with desquamation of the hands and feet are common symptoms. Management of shock,
renal failure, and adult respiratory distress syndrome (ARDS) are a priority if these
conditions are present, in addition to appropriate antibacterial treatment.
C. perfringens(choice A) causes gas gangrene, with necrosis of soft tissues, usually after a
traumatic wound. It is also a cause of food poisoning.
Although HIV-1 (choice B) can cause many diverse findings and should never be immediately ruled
out, the findings in this patient are most specific for TSS.
Shigella dysenteriae(choice C) is a cause of dysentery characterized by fever, abdominal
cramps, and bloody diarrhea.
S. epidermidis(choice E) is part of the normal skin flora, but is notorious for causing
infections of intravenous lines and prosthetic heart valves.

Explanation
The correct answer is A. The clinical signs suggest that this woman had hepatitis B three years
ago and the fact that she is now feeling better also suggests that she recovered from this
infection. Since she is pregnant, it is necessary to find out if she still has the organism in
her liver (chronic hepatitis B infection) by performing a test for HBsAg. This test will be
negative if she has completely recovered from the disease, but it would be positive if she is a
chronic carrier.
The clinical signs of the disease she had 3 years ago do not match those of CMV (choice B), and
CMV does not produce chronic infections.
Measurement of IgM antibody to hepatitis B core antigen (choice D) would be of no value at this
time. This antibody is positive in acute cases of hepatitis B, but would no longer be positive
in this case. Measurement of IgM anti-hepatitis B core antigen is one of the most important
tests in the hepatitis profile because the appearance of the antibody correlates with the
disappearance of HBsAg.
Examination of IgM antibody to HBsAg (choice C) would be of no value at this time. This IgM
antibody would be formed early during the recovery from acute hepatitis B, and would class
switch to IgG later on in the disease.
Quantitation of hepatitis A virus (HAV) IgM antibody (choice E) would be useless, since
hepatitis A virus (HAV) only causes acute infection.

Explanation
The correct answer is C. A single dose of a prophylactic antibiotic given as surgery is about
to begin (sometimes given intravenously to assure the timing) has become more common because
it is associated with a decreased wound infection rate. Such antibiotic usage does not prevent
organisms from entering the tissues, but effectively prevents them from becoming established
as they try to grow and divide (e.g., leave the lag phase (choice B) and enter the log or
exponential phase of colony growth). The stationary phase (choice E) and phase of decline
(choice D) (formerly called death phase, choice A) would occur much later, after a colony had
been established (which is what the surgeons were trying to prevent).

Explanation
The correct answer is C. This girl has streptococcal pharyngitis. The infecting organism is
group A beta-hemolytic streptococcus (S. pyogenes); its growth is inhibited by the placement of
a bacitracin disk on the throat culture plate. (Beta-hemolysis occurs as the result of the
bacterial hemolysin streptolysin S). This is a gram-positive bacterium and therefore possesses
a very thick peptidoglycan layer that would protect it from lysis by detergents. (Note that
gram-positives also contain teichoic acid.) In contrast, gram-negatives have a thin
peptidoglycan layer.
Keratin-like proteins in the spore coat (choice A) and calcium ion chelators (dipicolinic acid)
(choice E) are found in spores formed by species of Bacillus and Clostridium. These protect the
spores from the elements: dehydration, heat, chemicals, radiation, etc.
Lipopolysaccharide in the outer membrane (choice B) and a periplasmic space (choice D) are
found in gram-negative organisms. The lipopolysaccharide is an endotoxin and the periplasmic
space contains beta-lactamase in some species

Explanation
The correct answer is B. Bacterial growth in a closed system is characterized by four phases:
(1) In the initial lag phase, no growth occurs as the organisms adapt to the new environment.
(2) In the exponential, or log phase, the organisms grow at the fastest rate and antibiotics
that interfere with cell growth or division are most likely to be effective.
(3) In the stationary phase, when nutrients have been largely exhausted, organisms tend to
stop growing but may remain viable for long periods of time.
(4) In the phase of decline, cell deaths increase due to cell starvation or exposure to
toxins.

Explanation
The correct answer is C. The description of the agent is consistent with a staphylococcal
organism (catalase-positive, gram-positive cocci that grows on mannitol salt agar. The organism
is most likely S. aureus, as it was able to ferment mannitol (as evidenced by the color change
in the mannitol salt agar after incubation) and was beta-hemolytic. Other characteristics of
this organism are coagulase production and excretion of DNAse from colonies. Staphylococcal
organisms are also notorious for formation of abscesses; the patient had an abscess in the
tonsillar region.
The diphtheria bacilli (Corynebacterium diphtheriae; choice A) are gram-positive, pleomorphic
rods that are arranged in palisades. They are non-hemolytic and would not grow on mannitol salt
agar. These organisms produce a whitish-gray pseudomembrane on the pharynx or the larynx;
constitutional symptoms from toxemia are the major features of diphtheria.
Epiglottitis is the most common disease of the upper respiratory tract produced by Haemophilus
influenzae(choice 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.
Streptococcus salivarius(choice D) is a gram-positive coccus that is usually alpha-hemolytic.
It is normal flora of the oral cavity and is sometimes implicated in subacute bacterial
endocarditis, but is not associated with tonsillar abscesses.
Streptococcus pyogenes(choice E) is a beta-hemolytic, gram-positive coccus that grows in
chains, as opposed to the random, grape-like clusters of the staphylococci. These organisms are
the most common cause of pharyngitis, which is usually manifested by severe sore throat, fever,
a beefy red pharynx, and a tonsillar exudate.

Explanation
The correct answer is D. This is a case of Streptococcus pneumoniae meningitis. This organism
is able to attach to the respiratory mucosa because it has teichoic acids in its envelope, and
because it produces an IgA protease. The IgA protease physically cleaves the immunoglobulin
molecules, leaving the Fc parts to coat the bacterium. This allows the organisms to bind to the
Fc receptors on various mucosal cells.
Streptococcus pneumoniae does not produce a C carbohydrate (choice A), and thus cannot be
grouped by the Lancefield terminology.
Although the organism does produce the pneumolysin, which is an alpha-hemolytic exotoxin
(choice B), this toxin mediates damage to the respiratory epithelium and inhibition of
leukocytic responses, not attachment to the mucosa.
Streptococcus pneumoniae does not produce an hyaluronic acid capsule (choice C); Streptococcus
pyogenes does. Most capsules serve to inhibit phagocytosis, rather than to mediate adhesion.
Only the group A Streptococci possess an M protein (choice E), which is used to "type" them and
helps inhibit phagocytosis.

Explanation
The correct answer is E. Spore formation usually occurs during the stationary phase, when cell
growth ceases because of a developing lack of nutrients or accumulation of toxins.
During the phase of decline (choice A), the lack of nutrients and the accumulation of toxin
become so severe that any viable organisms usually die before they can form spores.
The exponential phase (choices B) and log phase (choice D) are descriptors for the steady state
of active growth occurring after the lag phase and before the stationary phase. Many
antibiotics are most effective in this period.
The lag phase (choice C) is the initial period of adaptation, prior to growth, which occurs
when organisms are introduced to a new environment.

Explanation
The correct answer is A. Amphotericin B is the most appropriate drug listed for the treatment
of cryptococcal meningitis. It is a polyene antibiotic that binds to ergosterol in the fungal
cell membrane, creating an artificial pore. Flucytosine is often prescribed as an adjunct
medication. Fluconazole is used long-term to prevent recurrence in AIDS patients.
Isoniazid (choice B) inhibits the biosynthesis of mycolic acids in the mycobacterial cell wall.
It is the primary drug used against tuberculosis. It is used alone for TB prophylaxis and is
used in combination with other antituberculars to treat patients with active disease.
Ketoconazole (choice C) is an orally administered imidazole antifungal medication. It inhibits
14-alpha-demethylase to block the synthesis of fungal cell membrane ergosterol. Note the
difference in mechanism between the polyenes, which alter ergosterol structure, and the
imidazoles, which block ergosterol synthesis. Ketoconazole is often used to treat
coccidioidomycosis (prevalent in California), histoplasmosis (prevalent in the Midwest),
blastomycosis (prevalent in the Eastern US), paracoccidioidomycosis (prevalent in Latin
America), and mucocutaneous candidiasis.
Metronidazole (choice D) is an antiprotozoal drug useful in treating a variety of parasitic
infections. It is the drug of choice for trichomoniasis and giardiasis and provides general
anaerobic coverage. This makes it useful for treating postsurgical abdominal and pelvic
Bacteroides fragilis infections or flare-ups of intestinal diverticulitis.
Nystatin (choice E) is an antifungal polyene that's usually used topically but can be taken
orally for oral and esophageal candidiasis. Candidal infections of the skin, mucous membranes,
and vagina usually respond well to this drug. It may also be used to prevent intestinal fungal
overgrowth in patients on chemotherapy

Explanation
The correct answer is C. The clinical and radiologic characteristics of this patient's
condition are consistent with lobar pneumonia, a respiratory infection that, in its classic
presentation, involves a single pulmonary lobe. In 90% to 95% of cases, the etiologic agent is
Streptococcus pneumoniae (AKA pneumococcus), an alpha-hemolytic streptococcus present in the
throat of 40% to 70% of healthy individuals. A small minority of cases of lobar pneumonia are
due to Klebsiella pneumoniae, staphylococci, streptococci other than pneumococcus, Haemophilus
influenzae, Pseudomonas, and Proteus. Penicillin is the drug of choice for pneumococcal
pneumonia, but sputum cultures are necessary to identify the infectious agent and determine its
antibiotic sensitivity. S. pneumoniae is a major cause of purulent meningitis in the elderly.
Other alpha-hemolytic streptococci (viridans streptococci) cause subacute endocarditis in
patients with previously altered cardiac valves. Streptococcus mutans and other oral
streptococci have been associated with dental caries.
Bacteria belonging to the genera Haemophilus(choice A) and Klebsiella (choice B) cause
respiratory infections; however, in immunocompetent individuals, these bacterial infections
usually result in bronchopneumonia, which leads to multilobar, and often bilateral, pulmonary
involvement.
Beta-hemolytic streptococci (choice D) cause countless infections in humans. Recall that
classification of β-hemolytic streptococci is based on their surface antigens known as
Lancefield antigens. Human diseases caused by this group of bacteria include:
- Streptococcus pyogenes (group A): pharyngitis, scarlet fever, erysipelas, impetigo, rheumatic
fever, and glomerulonephritis
- Streptococcus agalactiae (group B): neonatal sepsis and urinary infections
- Enterococcus faecalis (group D): endocarditis and urinary infections
Gamma-hemolytic streptococci (choice E) are streptococci that do not produce hemolysins
(nonhemolytic streptococci) and are not a significant cause of human disease.

Explanation
The correct answer is B. This patient has Rocky Mountain spotted fever. This diagnosis can be
confirmed with the immunohistologic demonstration of R. rickettsiae in the skin biopsy. The
tetracyclines are a class of antibiotics commonly used in the treatment of rickettsial
infections, acne, and various sexually transmitted diseases, as well as in the treatment of
infections caused by susceptible organisms in penicillin-allergic patients. The key to this
question is knowing which tetracycline antibiotic is safest to administer to a patient with
renal dysfunction. Doxycycline is secreted in an inactive form into the intestinal lumen and
eliminated in the feces; therefore, its half-life is largely independent of renal or hepatic
function. The other tetracyclines listed are concentrated by the liver in the bile and excreted
in the urine and feces unchanged. Therefore, dosage adjustments need to be made in patients
with renal impairment because of accumulation of the drug in the body. In other words,
demeclocycline (choice A), methacycline (choice C), oxytetracycline (choice D), and
tetracycline (choice E) will accumulate in patients with renal impairment.

Explanation
The correct answer is D. This patient has cryptococcal meningitis, as evidenced by the
"encapsulated organisms observable with India ink" in the CSF (a classic clue). Cryptococcus is
a nondimorphic yeast, meaning that it exists only in the yeast form. It is encapsulated (that's
why the India ink stain works so well) and it reproduces by budding. It is found worldwide in
bird droppings (think pigeons). It can also cause transient pulmonary illness in otherwise
healthy individuals.
The methenamine silver stain (choice A) is used primarily to demonstrate Pneumocystis carinii
in tissues.
Branching septate hyphae (choice B) are characteristic of Aspergillus fumigatus, among other
fungi.
Mycoses that exist in mycelial and yeast forms (dimorphism; diphasic; choice C) are Histoplasma
capsulatum, Coccidioides immitis, Blastomyces dermatidis, and Sporothrix schenckii.
C. neoformans is not dimorphic and it has a capsule (choice E).

Explanation
The correct answer is E. The disease described is measles (rubeola), which has the typical
presentation described in the question stem. Measles is caused by a Morbillivirus, an RNA virus
belonging to the Paramyxovirus family. Koplik's spots, which are pathognomonic for measles, are
small, bluish-white spots on the buccal mucosa in the early stages of measles. These lesions
appear just before the onset of the characteristic rash (which can also involve the
extremities) and fade as the rash develops.
Leukoplakia is a premalignant condition characterized by adherent whitish patches on the
gingiva (choice A) and other sites in the oral cavity.
Cold sores of the lips (choice B) are due to infection with herpes viruses.
Candida infection (thrush) produces curdy white material loosely attached to an erythematous
base (choice C).
Aphthous ulcers are large shallow ulcers of the oral mucosa (choice D), commonly known as
canker sores.

Explanation
The correct answer is C. Babesiosis is a syndrome characterized by malaise, fatigue, chills,
fever, myalgia, and arthralgia lasting for weeks. The protozoans Babesia microti and Babesia
divergens cause most human babesiosis. The organisms parasitize red blood cells, and have been
transmitted by blood transfusions. Hemolytic anemia and hepatosplenomegaly are features of
infection with these organisms; asplenic persons may have a life-threatening infection.
In Lyme disease, a characteristic rash (choice A), known as erythema chronicum migrans, forms
at the tick-bite site. In ehrlichiosis and babesiosis, low platelet counts may produce
nonspecific purpuric lesions, but there is not characteristic rash.
Arthritis of the knee (choice B) is a classic sign in untreated Lyme disease caused by Borrelia
burgdorferi.
Macrophages containing intracellular organisms (choice D) is the classic finding in multiple
tissues infected with Ehrlichia chaffeensis.
Ixodid ticks or hard-bodied ticks (choice E) are vectors for Babesiosis, Lyme disease, and
human granulocytic ehrlichiosis.

Explanation
The correct answer is E. The disease is poliomyelitis. Most infections with poliovirus cause
only the influenza-like symptoms, but a small percentage progress to paralytic poliomyelitis.
The most common causes of death are aspiration and airway obstruction as a result of bulbar
paralysis and paralysis of respiratory muscles. Arrhythmias can also be life-threatening.
Acute renal failure (choice A) is usually not seen in poliomyelitis, although the bladder may
become paralyzed.
Bowel paralysis (choice B) can be seen, but is not usually life-threatening.
Fulminant liver failure (choice C) is not a feature of poliomyelitis.
Gastrointestinal bleeding (choice D) can be seen in poliomyelitis, but is not usually lifethreatening.

Explanation
The correct answer is C. Nuchal rigidity suggests meningitis. It is convenient to classify
meningitis based on the cerebrospinal fluid (CSF) findings: 1) acute pyogenic meningitis if
neutrophils are markedly increased; 2) acute lymphocytic meningitis if lymphocytes (alone) are
markedly increased, and 3) chronic meningitis if lymphocytes, plasma cells, macrophages, and
fibroblasts are increased. This patient has chronic meningitis. The classical cause of chronic
meningitis is tuberculosis, whose etiologic agent is Mycobacterium tuberculosis. Other causes
include other indolent meningeal infections such as syphilis, brucellosis, and chronic fungal
infections. The granulomas that are characteristic findings in other tissues may or may not be
present in the meningeal tissue, and are usually not recognizable in CSF. Tubercular meningitis
is now uncommon in this country. In immigrants from third world countries, a history of
pulmonary tuberculosis may be distant, undiagnosed, or deliberately concealed.
Herpes virus (choice A) and mumps virus (choice B) are causes of acute lymphocytic meningitis.
Neisseria menigitidis(choice D) and Streptococcus pneumoniae(choice E) are causes of acute
pyogenic meningitis.

Explanation
The correct answer is D. The clues suggesting infection with Pseudomonas aeruginosa are the
green-tinged sputum (due to the formation of blue and green pigments by P. aeruginosa), and the
association with cystic fibrosis. Unfortunately, P. aeruginosa is very difficult to treat with
most antibiotics, and presents a very difficult therapeutic challenge in the management of
cystic fibrosis patients. Klebsiella pneumoniae(choice A) is most often seen in alcoholics, the elderly, and diabetics.
Mycoplasma pneumoniae(choice B) is a common cause of community-acquired atypical pneumonia, and
is characterized by a dry, non-productive cough.
Pneumocystic carinii(choice C) usually causes pneumonia in immunocompromised patients, e.g.,
AIDS patients.
Streptococcus pneumoniae(choice E) is an important cause of typical community-acquired lobar
pneumonia.

Explanation
The correct answer is G.Pseudomonas aeruginosa is a very common opportunist in burn patients,
in whom it classically causes secondary wound infections and septicemia. It may also cause
cystitis in patients with urinary catheters and pneumonia in patients with cystic fibrosis. The
organism is found in water and usually gains access to the body via this source, as a
contaminant in the water used in respirators or in water baths, etc. used to cleanse wounds.
This organism is a non-fermenter, that is, it does not metabolize sugars by classic pathways.
It produces a blue-green, water-soluble pigment (pyocyanin), and has a fruity odor when growing
on laboratory media. It has a propensity for developing antibiotic resistance; current therapy
employs the synergistic combination of an aminoglycoside, such as amikacin, with a cell wall
synthesis inhibitor (carbenicillin, ticarcillin, or piperacillin).
Candida albicans(choice A) is a normal flora yeast that will appear as very large, grampositive,
spherical-to-ovoid organisms with budding daughter cells in Gram-stained
preparations. Candidiasis is an opportunistic infection in individuals with a compromised
immune system. The fungus usually causes mucocutaneous lesions, but in severely compromised
individuals like AIDS patients, systemic disease may occur. Oral candidiasis appears as creamy,
white patches of exudate that can be scraped off an inflamed tongue or buccal mucosa.
Clostridium perfringens(choice B) is a gram-positive, spore-forming, anaerobic rod. It is a
common cause of gas gangrene when it is introduced into a wound. The organism produces a
variety of toxins and enzymes that enable it to destroy muscle tissue and spread through the
soft tissues of the body.
Escherichia coli(choice C) is a lactose-fermenting, gram-negative rod commonly seen as normal
flora of the intestine of man. It is the most common cause of urinary bladder infections,
pyelonephritis, and sepsis in patients with indwelling urinary catheters. It is also the major
cause of traveler's diarrhea with watery stools and is a very important pathogen in neonates,
who become infected during passage through the birth canal.
Klebsiella pneumoniae(choice D) is a gram-negative, highly encapsulated rod that is a
significant pulmonary pathogen in individuals with a compromised respiratory apparatus. It is a
common cause of aspiration pneumonia and pulmonary abscesses in alcoholics and patients with
chronic obstructive pulmonary disease. The organism is readily grown on standard laboratory
media such as blood agar or MacConkey's enteric agar.
Proteus mirabilis(choice E) and P. vulgaris(choice F) are highly motile, gram-negative rods
that ferment glucose, but not lactose. Other distinguishing features include "swarming" growth
on solid media (due to the high degree of motility and production of urease and hydrogen
sulfide). These organisms are not commonly associated with wound infections but usually cause
urinary tract infections. The ability to break down urea is thought to contribute to the
development of struvite kidney stones due to the elevation of urine pH by production of
ammonia.
Serratia marcescens(choice H) is a gram-negative organism that is found in the environment in
water, soil, and, occasionally, as normal flora of humans. It is an opportunistic pathogen that
causes respiratory disease in hospitalized patients. Many strains produce a pigment, but the
colonies are usually red, pink, or orange. Interestingly, the pathogenic varieties are most
often non-pigmented.

Explanation
The correct answer is D. Respiratory syncytial virus is the most common cause of bronchiolitis
and pneumonia in children younger than 1 year. Outbreaks occur seasonally in winter and early
spring. Infection does not result in lasting immunity, and reinfection can occur.
Coronavirus (choice A) causes the common cold (nasal obstruction and discharge, sneezing, no
fever or mild fever, occasional sore throat, and/or cough) and acute pharyngitis (sore throat,
with or without cervical adenopathy, ulceration, and conjunctivitis).
Influenza type A (choice B) is the leading cause of influenza. Influenza is a systemic illness
characterized by the sudden onset of fever, headache, myalgias, malaise, and prostration,
followed by cough, nasal obstruction, and sore throat. The lower respiratory tract may also be
involved.
Parainfluenza viruses (choice C) are the leading cause of croup, or acute
laryngotracheobronchitis, in children. This infection involves both the upper and lower
respiratory tracts. Inflammation in the subglottal area leads to hoarseness, dyspnea, a barking
cough, and inspiratory stridor.
Rhinovirus (choice E) is the most common cause of the common cold.

Explanation
The correct answer is D. Staphylococcal meningitis is fairly rare, occurring mostly in patients
with indwelling ventricular-peritoneal shunts. Staphylococcus epidermidis, which normally
colonizes the skin, is the most common organism causing this disorder; Staphylococcus aureus
meningitis occasionally occurs.
Bacteroides fragilis(choice A) is a common cause of anaerobic infections, including sepsis and
peritonitis, but it does not commonly cause meningitis.
Diphtheroids (including Corynebacterium diphtheriae, choice B) are sometimes isolated from
indwelling shunts, but in many cases they are simply contaminants, since some diphtheroids are
normal skin flora.
Escherichia coli(choice C) is isolated from 30% to 50% of neonates with bacterial meningitis,
but is not typically associated with shunt infections.
Streptococcus pneumoniae(choice E) is the most common cause of bacterial meningitis in people
older than 30 years; it is much less common in children.