A 55-year-old man returns from a vacation to the beach. While on vacation, he and his family consumed several meals consisting primarily of seafood, including lobster and raw oysters. Although the other members of his family did not get sick, he developed a bullous rash over his lower extremities, hypotension, and confusion. His past medical history is significant for liver disease and alcoholism. The most likely cause of this man's signs and symptoms is
E. Vibrio vulnificus
The correct answer is E.Vibrio vulnificus septicemia is contracted by consuming raw oysters. Patients with
chronic renal insufficiency, liver disease, hematopoietic disorders, and a past history of alcoholism are
particularly at risk. His past medical history makes him more susceptible to septicemia than are his family
members. Septicemia cause by Vibrio vulnificus generally begins with chills, fever, and hypotension, and skin
lesions tend to occur 24-48 hours after the onset of the infection.
Aspergillus fumigatus(choice A) is an opportunistic mold responsible for infections of wounds and burns in
immunocompetent hosts; in immunocompromised hosts, it can invade visceral organs such as the lungs.
Campylobacter jejuni(choice B) are curved, gram-negative rods that cause enterocolitis with diarrhea, and, less
commonly, chronic gastritis.
Candida albicans(choice C) is a common opportunistic yeast that is found as normal flora of upper respiratory,
gastrointestinal, and vaginal mucosae. It ordinarily causes thrush and vulvovaginitis; it may disseminate in the
Group A streptococcus (Streptococcus pyogenes; choice D) is an important cause of bacterial pharyngitis
A 24-year-old female has fever, malaise, and a dry, nonproductive cough. She also complains of headache, muscle aches, and leg pain. Lab values are significant for elevated cold agglutinins.
Which of the following microorganisms is responsible for her symptoms?
D. Mycoplasma pneumoniae
The correct answer is D.Mycoplasma pneumoniae is a wall-less bacterium that causes interstitial pneumonia in
young adults. Elevated cold agglutinins (a classic clue) are found in about half of the patients. The cold
agglutinins are IgM antibodies. M. pneumoniae may be diagnosed by sputum or complement fixation.
Haemophilus(choice A) causes bronchopneumonia in babies and children, and may occur in debilitated adults.
Klebsiella(choice B) causes a bronchopneumonia with patchy infiltrates involving one or more lobes. Think red
currant jelly sputum. It frequently occurs in debilitated patients, diabetics, and alcoholics. Note that these
organisms are highly encapsulated and produce mucoid colonies on lab media.
Legionella(choice C) lives in contaminated water sources such as air conditioning systems.
Streptococcus pneumoniae(choice E) is the most common cause of pneumonia in the elderly population and in
those with poor nutrition. It may lead to a pleural abscess and rusty-colored sputum; it is also a common cause
of sepsis and meningitis in the elderly
A Native American man is brought to a rural hospital in New Mexico. On arrival, he is unconscious with severe bronchopneumonia. Family members state that he suffered the sudden onset of chills, fever, and headache several days ago. One day later, the man complained of chest pain and difficulty breathing, and coughed up blood-tinged sputum. Chest x-ray reveals patchy infiltrates and segmental consolidation. Which of the following organisms is the most likely cause of this man's pneumonia?
E. Yersinia pestis
The correct answer is E. Any previously healthy person in the Southwestern United States who develops septic
shock or severe pulmonary disease should be evaluated for plague. Plague is not an extinct disease, but is still
encountered in sporadic cases in various places in the world, including Asia, Africa, parts of Europe, and the
American Southwest. The causative organism is Yersinia pestis, which is endemic in many wild animal
populations, and can be transmitted to humans either by direct contact or by arthropod bite. Human plague may
take many forms, including pestis minor (mild lymphadenopathy); bubonic plague (prominent
lymphadenopathy); pneumonic plague (as in this patient); and septicemic plague. The primary pneumonic form
typically presents as described. Antibiotics are most effective if given within the first 24 hours, which can be
problematic if medical staff do not suspect the disease. Since plague is rare in the United States, a high degree
of clinical suspicion is required to make a rapid diagnosis and to institute timely treatment. If the diagnosis is
missed, the mortality rate is quite high.
Brucella abortus(choice A) causes brucellosis, characterized by undulating fever, lymphadenopathy, and
Clostridium perfringens(choice B) causes gas gangrene and gastroenteritis.
Francisella tularensis(choice C) causes tularemia, associated with a spectrum of manifestations from an
influenza-like syndrome to adenopathy with ulceration at the site of inoculation.
Listeria monocytogenes(choice D) causes listeriosis. Infection during pregnancy may result in sepsis, abortion
or premature delivery. Infection in the neonate may produce meningitis. In immunocompromised adults, either
meningitis or sepsis may occur.
An elderly patient has pneumococcal pneumonia. She has no known drug allergies. Which of the following antibiotics would be most appropriate?
The correct answer is D. Penicillin remains the first-line drug of choice for pneumococcal pneumonia, except in
those patients with penicillin allergy and in the relatively few areas in which pneumococcal strains with high-level
penicillin resistance are found. Alternative therapies include erythromycin and vancomycin.
Chloramphenicol (choice A) is not usually used for pneumococcal pneumonia.
The third generation cephalosporin cefotaxime (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.
A 30-year-old veterinarian visits her obstetrician for a first-trimester prenatal check-up. She has no complaints. Routine physical exam is significant only for mild cervical lymphadenopathy. She is prescribed spiramycin but is noncompliant. Her baby is born with hydrocephalus and cerebral calcifications. Which of the following organisms is most likely responsible?
D. Toxoplasma gondii
The correct answer is D. Humans become infected with Toxoplasma gondii by ingesting cysts in contaminated
food or through contact with cat feces. The veterinarian in question was therefore particularly at risk of infection.
T. gondii is especially hazardous in pregnant women because the organism can be transmitted to the fetus
through the placenta. (It is part of the ToRCHeS group of congenital infections–Toxoplasma, Rubella,
CMV, Herpes/HIV, Syphilis). Since infected mothers are usually asymptomatic, cases often go unnoticed.
Occasionally, patients present with cervical lymphadenopathy, as did the veterinarian, and require treatment to
prevent complications in the fetus. Though newborns are also often asymptomatic, they are at risk for
developing the classic triad of chorioretinitis (at birth or later in life), hydrocephalus, and cerebral calcifications.
Note that T. gondii is also a common cause of CNS infections (e.g., encephalitis) in HIV-positive patients.
Isospora belli(choice A) is an intestinal protozoan that causes watery diarrhea, particularly in the
immunocompromised. Fecal-oral transmission of oocysts allows invasion of small intestinal mucosa, destroying
the brush border.
Leishmania donovani(choice B) causes kala-azar (visceral leishmaniasis), which is characterized by fever,
weakness, weight loss, splenomegaly, and skin hyperpigmentation. It is prevalent in regions of the
Mediterranean, Middle East, Russia, and China. The vector is the sandfly.
Plasmodium vivax(choice C) causes malaria and is transmitted by the female Anopheles mosquito, which
introduces sporozoites into the blood. These differentiate into merozoites that destroy erythrocytes.
Splenomegaly ensues. Other species of this organism also cause malaria: P. malariae and P. falciparum (which causes a more severe form of the disease). Note that sickle cell trait confers resistance to this disease.
Trypanosoma cruzi(choice E) causes Chagas' disease, characterized by facial edema and nodules, fever,
lymphadenopathy, and hepatosplenomegaly. It affects cardiac muscle most severely and is a major cause of
cardiac disease worldwide. The reduviid ("kissing") bug is the vector that infects humans through bites. It is most
prevalent in Central and South America, with rare cases in the southern US.
A 30-year-old veterinarian on a cattle ranch presents with a 1-to-2-month history of malaise, chills, drenching malodorous sweats, fatigue, and weakness. He has anorexia and has lost 15 pounds. He has intermittent fevers that range up to 103 F (39.4 C). He complains of visual blurring. A physical examination reveals mild lymphadenopathy, petechiae, and a cardiac murmur consistent with aortic insufficiency. What is the most likely etiologic agent?
B. Brucella abortus
The correct answer is B.Brucella abortus produces a chronic, granulomatous disease with caseating
granulomas. Most cases occur in four states (Texas, California, Virginia, and Florida), and are associated with
cattle, in which it produces spontaneous septic abortions. Most cases of brucellosis produce mild disease or
fevers of unknown origin. However, Brucella spp. can infect the cardiovascular system and cause a localized
infection. B. abortus is the most common species to cause endocarditis. The aortic valve is most commonly
involved, followed by the mitral valve, and then both valves. Most cases of brucellosis are associated with
occupational exposure, in persons such as veterinarians, ranchers, and those who handle carcasses.
Bacillus anthracis(choice A) is the causative agent for anthrax. It usually produces cutaneous disease
(malignant pustule or eschar) at the site of inoculation in handlers of animal skins. It can also produce a severe
hemorrhagic pneumonia (Woolsorter's disease) and septicemia. At-risk groups include those who handle animal
carcasses or skins.
Coccidioides immitis(choice C) is a dimorphic fungal disease producing a granulomatous pulmonary syndrome
that is more severe in dark-skinned individuals. Disseminated disease occurs most often in Filipinos, Mexicans,
and Africans. The infective form is the arthrospore; the diagnostic form in tissue is the spherule containing
endospores. The disease is endemic in the San Joaquin River Valley. At-risk groups include military personnel,
agricultural workers, construction workers, oil field workers, archaeology students, participants in outdoor
sports, and sightseers. Remote infections from fomites (cotton harvested in the Southwestern U.S.) have been
Erysipelothrix rhusiopathiae(choice D) is a pleomorphic, gram-negative rod that causes a localized skin
infection. It is an occupational disease of fishermen, fish handlers, butchers, meat-processing workers, poultry
workers, farmers, veterinarians, abattoir workers, and housewives.
Trichinella spiralis(choice E) is a nematode infection caused by the ingestion of larvae found in undercooked
meat. Pork is the most common contaminated meat. However, outbreaks in the northern parts of the U.S. have
been associated with eating undercooked infected bear meat. Symptoms include diarrhea, periorbital edema,
myositis, fever, and eosinophilia.
Six days after receiving several flea bites in a rat-infested shed in Southeastern New Mexico, a homeless 24-year-old-man develops fever, chills, and a rash that spreads from his abdomen to cover his extremities. He is seen at the hospital emergency department, where blood is drawn for analysis. Eight days later, the public health department reports the presence of antibody to one of the rickettsial group antigens.
Which of the following is the most likely diagnosis?
A. Endemic typhus
The correct answer is A. Endemic typhus is caused by Rickettsia typhi and is found worldwide. It is spread by
the feces of the rat flea, and its reservoir is the rat.
Epidemic typhus (choice B) is cause by Rickettsia prowazekii. It is prominent during times of war and social
upheaval and is spread by the feces of the body louse, Pediculus humanis. Humans are the chief reservoir,
although it is also found in populations of flying squirrels along the Atlantic coast of the U.S.
Q fever (choice C) is caused by Coxiella burnetii of the family Rickettsiaceae. Unlike the other rickettsial
illnesses, Coxiella burnetii is not usually transmitted to humans by the bite of an arthropod. Since the organism
is resistant to dehydration, it can be acquired by inhaling dust contaminated by animals with asymptomatic
infections, such as goats, sheep, and cattle.
Rocky Mountain spotted fever (RMSF) (choice D) is caused by Rickettsia rickettsii. It is transmitted by the bite
of an infected tick of the genus Dermacentor. Since the tick passes the infection transovarially to its progeny,
the tick is a reservoir. Rodents and other mammals are also reservoirs. Despite the name, most cases of RMSF
occur in the central states and southern Atlantic seaboard states. Oklahoma has the highest incidence of the
Scrub typhus (choice E) is caused by Rickettsia tsutsugamushi. It occurs mainly in Southeast Asia, where it is
transmitted by mites. Mites and rodents are the reservoirs for the organism.
A 54-year-old diabetic patient reports to his physician's office complaining of an unresolved skin lesion on his foot. The lesion began several weeks ago as a blister and has since become a painful, erosive, expanding sore. On examination, the affected site is now 5 cm in diameter, with a black necrotic center and raised red edges. Which of the following toxins has a mechanism of action most similar to the toxin responsible for tissue damage in this patient?
E. Diphtheria toxin
The correct answer is E. This patient's wound is infected with Pseudomonas aeruginosa, and the characteristic
lesion described is called ecthyma gangrenosum. The Pseudomonas alpha toxin, which is responsible for the
tissue damage, inhibits protein synthesis by acting on EF-2 with a primary target cell in the liver. The diphtheria
toxin has a similar action, although its target cells are heart and nerve.
Anthrax toxin (choice A) is an adenylate cyclase that causes fluid loss from cells.
Botulinum toxin (choice B) is a neurotoxin that decreases acetylcholine synthesis.
Cholera toxin (choice C) acts to increase adenylate cyclase activity by ribosylation of GTP-binding protein.
Clostridium perfringens alpha toxin (choice D) is a lecithinase.
Escherichia coli labile toxin (choice F) works in a fashion similar to the cholera toxin.
Pertussis toxin (choice G) causes fluid loss by ribosylating Gi.
Shigatoxin (choice H) decreases protein synthesis by inhibiting the 60S ribosomal subunit.
Streptococcal erythrogenic toxins (choice I) act similarly to the diphtheria toxin, but do so by increasing cytokine
Tetanus toxin (choice J) is a neurotoxin that inhibits the inhibitory neurotransmitters glycine and GABA.
TSST-1 (choice K) is a superantigen that acts by increasing cytokine production and decreasing liver clearance
An 18-year-old, previously healthy female presents to the student health service with fever, vomiting, and diarrhea. On physical examination, she is hypotensive and has an erythematous, red, sunburn-like skin rash. She is currently menstruating, and has been using super absorbent tampons.
Which of the following findings from a positive blood culture would confirm your suspected diagnosis?
B. Organisms are coagulase positive
The correct answer is B. This is a multi-step microbiology question that requires you to diagnose the illness,
identify the microorganism, and remember its key feature. The first part should be easy: everything about this
vignette suggests toxic shock syndrome. The organism in question is therefore Staphylococcus aureus, which is
coagulase positive. All of the other choices are classic features of other important pathogenic microorganisms:
Acid-fast organisms (choice A), refers to Mycobacteria. (In addition, Nocardia species are partially acid fast).
EMB agar (choice C) refers to a selective and differential medium used to isolate and identify enteric
gram-negative bacteria. Gram-positive bacteria will not grow on EMB agar because the addition of eosin inhibits
their growth. Nonlactose fermenters will have colorless colonies, while fermentation of this sugar will cause the
colonies to appear pink or purple.
Thayer-Martin media (choice D) is a growth medium for pathogenic Neisseria species. It contains the antibiotic
vancomycin, which kills gram-positive organisms such as Staphylococcus aureus.
The Quellung reaction (choice E) can be used to identify the capsule type of a microorganism. Encapsulated
microorganisms, like the Pneumococci and Haemophilus, are mixed with specific antisera. If the antiserum is
directed against the microorganism's capsule type, the capsule will be opsonized, absorb water, and become
visible under a light microscope.
A 24-year-old woman presents with a 3-day history of fever, chills, chest pain, and cough productive of rust-colored sputum. Past medical history includes a splenectomy 1 year ago. A chest x-ray film indicates consolidation of the right lower lobe. Blood cultures are positive for α-hemolytic gram-positive diplococci. Immunity to the causative organism is based on
E. IgG antibodies to a surface acidic polysaccharide
The correct answer is E. The patient in this question has pneumococcal pneumonia, which must be
considered in any patient with chills, fever, chest pain, and cough productive of purulent, rust-colored sputum.
Streptococcus pneumoniae is an α-hemolytic, gram-positive coccus that grows in chains. It can be easily
distinguished from other α-hemolytic streptococci because it is exquisitely sensitive to bile and bilelike
compounds, such as optochin. It is the most common cause of community-acquired pneumonia and the most
common cause of community-acquired meningitis in adults older than 30. The only recognized virulence factor
of S. pneumoniae is its carbohydrate capsule (which contains acidic polysaccharides). Antibody to a specific
capsule type is necessary to overcome infection. More than 80 capsule types have been recognized. The 23
types that most commonly cause disease are contained in a vaccine that is recommended for high-risk groups,
including the elderly and those undergoing splenectomy. Increased susceptibility is also found in patients with
Hodgkin disease, chronic lymphocytic leukemia, and myeloma. The vaccine should still be given to patients with
these conditions, but it is less successful.
The alternative complement pathway (choice A) is important in clearing Neisseria infections. Individuals with
deficiencies in C5 through C8 are at increased risk of disease from Neisseria.
The fimbria of Streptococcus pyogenes (Group A α-hemolytic streptococcus) is composed of an
α-helically coiled M protein. Antibody against a specific M type (choice B) will prevent infection. However,
raising antibodies to M proteins can lead to rheumatic fever, so Strep throat infections are routinely treated with penicillin to prevent an antibody response.
The C carbohydrate is an antigen of α-hemolytic streptococci used to divide them into different groups.
Antibody against C carbohydrate (choices C and D) is not protective.
A 3-year-old boy presents with a 1-day history of loose stools, fever, abdominal cramping, headache, and myalgia. He has no blood in the stool. A careful history reveals that he has several pet turtles. Which of the following is most likely the causative agent of his diarrhea?
C. Salmonella spp.
The correct answer is C. Salmonella spp., including S. enteritidis and S. typhimurium, produce a gastroenteritis
or enterocolitis. Patients with decreased gastric acidity, sickle cell disease, defects in immunity, or children
younger than 4 years have a more severe course of disease. Salmonella spp. are carried in nature by animal
reservoirs such as poultry, turtles, cattle, pigs, and sheep. The incubation period is 8-48 hours after ingestion
of contaminated food or water.
Chlamydia psittaci(choice A) produces an interstitial pneumonitis accompanied by headache, backache, and a
dry, hacking cough. A pale, macular rash is also found on the trunk (Horder's spots). Patients at risk include pet
shop workers, pigeon handlers, and poultry workers.
Entamoeba histolytica(choice B) produces a diarrhea (frequently bloody or heme-positive), right lower quadrant
crampy abdominal pain, and fever. Patients frequently have weight loss and anorexia. There is usually a history
of travel outside the U.S. Most cases are chronic. Complications include liver abscesses.
Staphylococcus aureus(choice D) produces a self-limited gastroenteritis due to the production of preformed,
heat-stable enterotoxins. The incubation period is 16 hours. The toxins enhance intestinal peristalsis and
induce vomiting by a direct effect on the CNS.
Yersinia enterocolitica(choice E) usually produces a chronic enteritis in children. These patients have diarrhea,
failure to thrive, hypoalbuminemia, and hypokalemia. Other findings include acute right lower quadrant
abdominal pain, tenderness, nausea, and vomiting. The infection mimics appendicitis or Crohn's disease.
Bilateral tonsillectomy is performed on an otherwise healthy 11-year-old female with recurrent upper respiratory tract infections. On sectioning the tonsils, numerous small, yellow granules are noted. A granule crushed between two slides has a dense, gram-positive center and numerous branching filaments at the periphery. The granules are most likely composed of which of the following organisms?
A. Actinomyces israelii
The correct answer is A. Actinomyces are normal inhabitants of the gastrointestinal tract that grow under
anaerobic and microaerophilic conditions. Although they are gram-positive rods, they grow as branching
filaments and have been confused with fungi. The yellow colonies (sulfur granules) are found in low-oxygen
niches like the tonsils and in actinomycotic abscesses.
Aspergillus fumigatus(choice B)may be present in the respiratory tract as an opportunistic pathogen; however,
fungus balls are generally seen only in pre-existing cavities (e.g., bronchiectasis, TB), not in the tonsils.
Blastomyces dermatitidis(choice C) is a respiratory pathogen that is seen as thick-walled yeasts within
Candida albicans(choice D), also a normal inhabitant of the oral cavity, would present as whitish plaques and
would appear microscopically as budding yeasts.
Diphtheria, caused by Corynebacteria diphtheriae(choice E), is a gram-positive rod. The disease presents with
a gray-white membrane in the oropharynx, and large colonies would not be appreciated.
A patient with a cavitary lung lesion coughs up sputum that contains thin, acid-fast positive rods. Which of the following features would most likely be associated with these bacteria?
E. Waxy envelope
The correct answer is E. Mycobacteria, such as the causative organism of this patient's tuberculosis, are "acid
fast" because they have an envelope that contains large amounts of lipids and even true waxes (unlike
envelopes of other types of bacteria) that prevents the acid-fast stain (carbolfuchsin) from leaking out.
The other characteristics listed in the answers are commonly tested features of specific bacteria:
Nutritional requirement for factors V and X (choice A) is a feature of Haemophilus influenzae.
Streptokinase (choice B) is a feature of Streptococci.
Toxic shock syndrome toxin (choice C) is a feature of Staphylococcus aureus.
Visibility under dark field illumination (choice D) is a feature of the syphilis organism Treponema pallidum.
An 8-year-old boy is brought to the emergency room with a 3-day history of fever to 102 degrees F and abdominal pain. He also complains of pain in his right knee and right elbow. He was seen four weeks ago because of a sore throat and a rash. A throat culture performed at that time grew gram-positive cocci in chains. Amoxicillin was prescribed, but the boy's mother did not fill the prescription. On physical exam his temp is 101.7F, HR 96, and BP 100/60. Cardiac exam reveals a pansystolic blowing murmur heard best at the apex. His right elbow is tender on extension and flexion with mild swelling. Laboratory tests reveal a positive C-reactive protein, an ESR of 40 and a WBC of 22,000 with a left shift. EKG shows a prolonged PR interval.
Which of the following tests would be positive for the microorganism responsible for this patient's illness?
C. Sensitivity to bacitracin
The correct answer is C. This case is classic for Rheumatic fever, including two of the major Jones criteria
(carditis, polyarthritis) and several minor criteria (fever, arthralgia, elevated ESR, leukocytosis, C-reactive
protein, prolonged PR interval). Rheumatic fever is a sequela of untreated infection with Group A
Streptococcus (S. pyogenes). S. pyogenes is differentiated from the other beta-hemolytic Strep by its sensitivity
to the antibiotic bacitracin.
The catalase test (choice A) is used to differentiate Staphylococci from Streptococci. Staphylococci are
catalase positive, Streptococci are catalase negative. So, S. pyogenes would be catalase negative.
The coagulase test (choice B) is used to differentiate Staph aureus from the other Staph spp. Staph aureus is
coagulase positive; the others are negative.
Sensitivity to novobiocin (choice D) is used to differentiate Staph saprophyticus (resistant) from Staph
Sensitivity to optochin (choice E) is used to differentiate Strep pneumoniae (sensitive) from viridans Strep
An immigrant from the Far East develops malaise, fever, and rigors, followed by upper right quadrant abdominal pain, vomiting, jaundice, and itching. His urine is dark and his feces are pale. Infestation with which of the following parasites is most strongly suggested by this patient's presentation?
A. Clonorchis sinensis
The correct answer is A. The patient is suffering from suppurative (bacterial) cholangitis, which can occur as a complication of infestation by the roundworm, Ascaris lumbricoides and by the liver flukes, Clonorchis sinensis
and Fasciola hepatica. Biliary tract obstruction produces jaundice and extreme itching, with dark urine and pale
feces. Therapy typically includes emergency endoscopic sphincterectomy to improve biliary drainage,
antibiotics, and anthelminthic agents.
Enterobius vermicularis(choice B) causes pinworm infections.
Plasmodium ovale (choice C) causes malaria.
Taenia solium (choice D) is the pork tapeworm. Adult tapeworms cause taeniasis, while the larvae are
responsible for cysticercosis.
Trypanosoma cruzi(choice E) causes Chagas' disease.
A 38-year-old woman vacationing in Connecticut is bitten by a tick. She does not seek medical treatment and eventually develops chronic arthritis of the knee and hip joints and paralysis of the left facial muscles. A physical examination during the early stages of the disorder would most likely have revealed
B. Erythema chronicum migrans
The correct answer is B. Lyme disease should be suspected in a patient who is bitten by a tick in the
northeastern U.S. Lyme disease was named after a township in eastern Connecticut where the disease was
endemic. The disease is spread via a tick vector of the genus Ixodes, which transmits a spirochete that causes
a systemic illness. Erythema chronicum migrans is usually the first sign of the illness. This is a large red patch
on the buttocks or chest that slowly expands as the center blanches. Generally, patients also have
constitutional symptoms, such as fever and chills, during this phase. Stiff neck may develop, along with other
signs of meningeal irritation, because of an aseptic meningitis. Other neurologic complications of Lyme disease
include Bell's palsy due to involvement of branches of the facial nerve. Arthritis is a prominent feature in about
half the patients with Lyme disease. It tends to appear several months after the infection but may persist for
several years. The course of the chronic arthritis shows exacerbations and remissions; the most commonly
affected joints are the knees and hips. Cardiac abnormalities in Lyme disease include pericarditis and heart
Skin manifestations do not include aphthous ulcers (choice A).
Flaccid or spastic paralysis of limbs (choices C and E) does not accompany Lyme disease; neurologic
involvement is generally limited to cranial nerves and meningitis.
Purpura (choice D) is associated with vasculitis and does not occur in Lyme disease.
Global eradication of Lyme disease is unlikely because
A. Borrelia burgdorferi can be maintained in nature indefinitely by a tick vector
The correct answer is A.Borrelia burgdorferi can be maintained in nature indefinitely by a tick vector. This
organism is the tick-transmitted spirochete that causes Lyme disease. The tick, Ixodes dammini, can infect both the white-footed mouse and large mammals such as deer during its life cycle, making them reservoirs. The tick itself is a reservoir, however, since it acquires the disease through transovarial passage of the organism. Together, these factors make Lyme disease an endemic infection with little hope for eradication.
Borrelia burgdorferi is not resistant to antibiotics and disinfectants (compare with choice B). The spirochete can be successfully treated with penicillins, tetracycline, and ceftriaxone.
Borrelia burgdorferi is a delicate spirochete and is not resistant to environmental stresses (compare with choice C).
Brill-Zinsser disease (choice D) is the reactivation of epidemic typhus infection caused by Rickettsia prowazekii. It can occur many years after an infection that was not treated with antibiotics.
Humans are incidental hosts, rather than the primary reservoir, for Borrelia burgdorferi(choice E). The primary reservoirs are ticks, mice, and large mammals.
IF you run out of limes, you can substitute lemons. By the time you start drinking tequilla most people are so drunk they wont notice the difference.
A 54-year-old farmer in rural Pennsylvania presents to his physician with chronic cough. Chest x-ray demonstrates a mass lesion with hilar lymphadenopathy. Biopsy of the mass demonstrates multiple, tiny yeast forms within macrophages.
Which of the following is the most likely diagnosis?
The correct answer is C. Histoplasmosis is caused by Histoplasma capsulatum, a dimorphic fungus that grows as
a mold in the wild, but as a tiny yeast inside macrophages in humans. The disease is typically asymptomatic or
mild enough to go undetected, but when symptomatic, it presents with cough, fever, and malaise.
Blastomycosis (choice A) is characterized by a larger, round-budding yeast form, seen free in the tissues.
Coccidioidomycosis (choice B) is seen mainly in the desert parts of the Southwest U. S. In the lungs, spherules
containing endospores are seen.
Paracoccidioidomycosis is endemic in Latin America, especially in Brazil. The infected cells show a typical "pilot's
wheel" appearance due to multiple yeasts sprouting out of a single parent cell (choice D).
Sporotrichosis (choice E) most often produces a localized cutaneous infection, following inoculation
occurring in association with minor skin trauma during gardening.
Two weeks after birth, a neonate develops sepsis, skin vesicles, and conjunctivitis. Over the next several days, the baby's condition deteriorates with development of seizures, cranial nerve palsies, and lethargy. The baby dies approximately one week after onset of symptoms.
Which of the following infectious agents would most likely cause this clinical presentation?
B. Herpes simplex
The correct answer is B. All of the agents listed, including choices A, C, D and E, can cause devastating
congenital infections with high mortality and often with major organ malformation (the TORCH agents:
Toxoplasma, other, rubella, cytomegalovirus, herpes simplex). However, it is herpes simplex type II, typically
acquired during delivery, that causes the devastating neonatal encephalitis described in the question stem. The
mortality rate for neonatal herpes is about 65%, and only 10% of the babies escape without neurologic
sequelae. A point worth remembering is that adult herpes encephalitis (in non-immunosuppressed individuals) is
usually due to herpes simplex I, while neonatal and congenital herpes are usually due to herpes simplex II.
A one-week-old baby develops nuchal rigidity and fever. A lumbar puncture is performed and the cerebrospinal fluid demonstrates large numbers of neutrophils.
Which of the following is the most likely causative agent?
B. Escherichia coli
The correct answer is B. The nuchal rigidity (stiff neck) suggest meningitis. Numerous neutrophils in the
cerebrospinal fluid suggests a bacterial pathogen. The best answer of those listed is Escherichia coli, which is
normally a gut organism, but can infect neonates who acquire the organism during passage through the birth
Coxsackievirus (choice A) is a cause of acute lymphocytic meningitis.
Herpes virus (choice C) is a cause of lymphocytic meningitis. Mycobacterium tuberculosis(choice D) is a cause of chronic meningitis.
Neisseria meningitidis(choice E) causes bacterial meningitis, usually in the second or third decade of life.
A patient presents to a physician because of pain during defecation accompanied by blood in the stool. Physical examination demonstrates a large perianal mass. Pathologic examination of the rectal mass following resection demonstrates a condyloma in which transformation to frank carcinoma has occurred.
Which of the following viruses would most likely be associated with these lesions?
D. Human papilloma virus (HPV)
The correct answer is D. The tumor is anal carcinoma arising in a condyloma. Both condyloma and anal
carcinoma are related to human papilloma virus (HPV), which is also associated with cervical and penile
condylomas and carcinomas.
EBV (choice A) is associated with Burkitt's lymphoma and nasopharyngeal carcinoma.
HBV (choice B) is associated with hepatocellular carcinoma.
HHV8 (choice C) is associated with Kaposi's sarcoma.
HTLV-1 (choice E) is associated with adult T-cell leukemia
A 23-year-old woman with a history of sickle cell disease presents with fever and severe bone pain localized to her left tibia. X-ray reveals a lytic lesion and blood cultures reveal infection. A bone culture grows gram-negative rods.
Which of the following best describes the infecting organism?
D. It is motile and does not ferment lactose
The correct answer is D. The presence of sickle cell disease in a question stem is usually a significant clue.
This question tests if you know that patients with sickle cell anemia are more susceptible to osteomyelitis
caused by Salmonella. (The patient's fever, bone pain, and x-ray results indicate osteomyelitis). But note that
Staphylococcus aureus (gram-positive coccus) is the most common cause of osteomyelitis in both sicklers and nonsicklers. If it had not been ruled out on bone culture, you should have looked for it in the answer choices.
Notice that you were required to know more than just the organism's name; you needed to know its
distinguishing features. Choice D describes Salmonella (a gram-negative rod) accurately. Salmonella exists in
over 1800 serotypes and is known to contaminate poultry.
A facultative intracellular parasite (choice A) is Legionella, a catalase-positive gram-negative rod. It
contaminates air-conditioning cooling towers and causes Legionnaire's disease (a type of pneumonia).
A nonmotile, facultative anaerobe (choice B) is Shigella, a gram-negative rod that does not produce H2S. All
Shigella contain an endotoxic lipopolysaccharide. The organism causes bacillary dysentery, with abdominal
cramps, fever, and mucoid, bloody diarrhea.
A comma-shaped organism that is sensitive to acidic pH (choice C) is Vibrio cholerae, a gram-negative rod that
causes severe enterotoxin-induced diarrhea, with "rice-water" stools and dehydration. The toxin acts by
stimulating adenylyl cyclase to overproduce cAMP in the brush border of the small intestine.
A motile and oxidase positive organism (choice E) is Pseudomonas, a gram-negative rod with pili that
sometimes produces a polysaccharide slime layer. P. aeruginosa is the prototype and commonly colonizes the
lungs of patients with cystic fibrosis. It is associated with blue-green pus.
A 42-year-old Hispanic man is brought to the emergency room by ambulance after suffering a grand mal seizure at home. There is no history of recent illness, fever, headache, seizures, or head trauma. He drinks alcohol occasionally and denies any other drug use. The patient has been a resident of the United States for 15 years, but occasionally travels to his previous home in Honduras.
Neurologic exam shows the patient to be alert and oriented. No focal abnormalities are noted. A CT scan of the head reveals multiple punctate calcifications, and two enhancing cystic lesions with surrounding edema. What is the most likely diagnosis?
The correct answer is D. Cysticercosis is a parasitic infection caused by the larval cysts of the tapeworm Taenia solium. Patients acquire the infection by ingesting the eggs, which reach a larval stage in various tissues. When the central nervous system (CNS) is involved, the condition is known as neurocysticercosis. It is the most common parasitic infection of the CNS. Infection with this organism is most frequently encountered in individuals from Mexico, South Central America, the Philippines, and Southeast Asia. In the CNS, the cysts act as space-occupying lesions and can cause hydrocephalus and/or seizures. Seizures are the most common initial presentation of patients with neurocysticercosis and may be focal or generalized. Signs of increased intracranial pressure such as headache, nausea, vomiting, or visual changes may also be present.
Amebiasis (choice A) is caused by Entamoeba histolytica. Patients typically present with diarrhea (often bloody), right lower quadrant abdominal pain, and fever. Amebic abscesses in the liver are a complication due to invasion of the portal venous system by the amoeba.
Cytomegalovirus (choice B) produces neonatal infections and infections in immunocompromised patients such
as AIDS patients. CD4 counts are usually
Which of the following viruses produce disease or sequelae that is/are more severe if the infection occurs at a very young age?
B. Hepatitis B virus
The correct answer is B. Infection with Hepatitis B virus (HBV) at birth or a very young age is
associated with chronic HBV infection and the development of hepatocellular carcinoma later in
life. In fact, infants born to Hepatitis B surface antigen (HBsAg)-positive mothers are
commonly infected, and approximately 90% become chronic carriers of the virus. Chronic carriers
suffer from hepatocellular carcinoma at an incidence over 200 times higher than a noncarrier.
The current recommendation for infants born of HBsAg-positive mothers is administration of
hepatitis B immunoglobulin (HBIg) in the delivery room, with the first dose of the hepatitis B
vaccine given at the same time or within 1 week. The second and third dose of the vaccine are
then given at 1 and 6 months. With this protocol, 94% protection is achieved.
The Epstein-Barr virus (EBV; choice A) is the agent of heterophile-positive infectious
mononucleosis. In children, primary EBV infection is often asymptomatic.
The measles virus (choice C) often causes a more severe disease in adults. Over the age of 20,
the incidence of complications, including pneumonia, bacterial superinfection of the
respiratory tract, bronchospasm and hepatitis, is much higher than in children.
Poliovirus (choice D) causes asymptomatic or inapparent infections 95% of the time. Frank
paralysis occurs in approximately 0.1% of all poliovirus infections. However, the probability
of paralysis increases with increasing age.
Varicella zoster virus (choice E) is the agent of chickenpox and shingles. In immunocompetent
children, it is a benign illness with a mortality of less than 2 per 100,000 cases. This risk
is increased over 15-fold in adults. Much of the increase is due to varicella pneumonitis, a
complication that occurs more frequently in adults.
A 40-year-old otherwise healthy gardener presents with several subcutaneous nodules on his right hand, where he had cut himself on rose thorns. Physical exam reveals several erythematous fluctuant lesions.
Which of the following organisms is most likely responsible for his condition?
The correct answer is E. Whenever you see a question on a gardener who works with roses, think
"Sporothrix schenckii." This organism is responsible for "rose gardener's disease," known
technically as sporotrichosis. The organism enters through skin breaks in the fingers or hands,
causing a chancre, papule, or subcutaneous nodule with erythema and fluctuance. Ulcerating
lesions appear along lymphatic channels, but the lymph nodes are not commonly infected.
Potassium iodide is the treatment of choice for the subcutaneous manifestations.
Aspergillus (choice A) causes pulmonary aspergillosis–a systemic mycotic infection. There
is an allergic type that is caused by a hypersensitivity reaction to the organism and an
infectious type that occurs more commonly in the immunocompromised. Hemoptysis is a common
Malassezia (choice B), specifically M. furfur, causes tinea versicolor, a superficial mycotic
Onchocerca (choice C), specifically O. volvulus, causes river blindness. This organism is a
helminth that is transmitted by black flies in Africa, Central America, and South America.
Rhizopus (choice D) causes rhinocerebral infections in diabetics with ketoacidosis.