Which of the following is a feature of gram-positive bacteria rather than gram-negative bacteria?
E. Thick peptidoglycan cell wall
The correct answer is E. Most of the features listed are those of gram-negative bacteria, which have a complex
cell envelope consisting of a cytoplasmic (inner) membrane, a periplasmic space containing peptidoglycan
(choice D) and lipoprotein (choice B), an outer membrane (choice C), and sometimes a capsule. The outer
membrane contains lipopolysaccharide (choice A) which is a major component of endotoxin. The peptidoglycan cell wall of the gram-negative bacteria is thin, while that of the gram-positive bacteria is thick. Other features of
gram- positive bacteria include a fairly simple surface with cytoplasmic membrane, peptidoglycan, cell wall, and
sometimes, an outer capsule. The cell wall contains lipoteichoic acids.
A mailman gets a severe bite wound from a pit bull guarding a junkyard. The wound is cleansed and he receives
a booster injection of tetanus toxoid and an injection of penicillin G. Several days later, the wound is inflamed and
purulent. The exudate is cultured on blood agar and yields gram-negative rods. Antibiotic sensitivity tests are
pending. The most likely agent to be isolated is
D. Pasteurella multocida
The correct answer is D. Pasteurella multocida is a gram-negative rod that is normal flora of the oral cavity of
dogs and cats. It often causes a local abscess following introduction under the skin by an animal bite. Most
cases occur in children who are injured while playing with a pet.
Bartonella henselae (choice A) is a very small, gram-negative bacterium that is closely related to the rickettsia,
although it is able to grow on lifeless media. It is the cause of cat-scratch disease (a local, chronic lymphadenitis
most commonly seen in children) and bacillary angiomatosis (seen particularly in AIDS patients). In this latter
patient population, the organism causes proliferation of blood and lymphatic vessels causing a characteristic
"mulberry" lesion in the skin and subcutaneous tissues of the afflicted individual.
Brucella canis(choice B) is a gram-negative rod that is a zoonotic agent. Its normal host is the dog, but when it
gains access to humans, it causes an undulating febrile disease with malaise, lymphadenopathy and
hepatosplenomegaly. The normal route of exposure is via ingestion of the organism.
Clostridium tetani(choice C) is a gram-positive spore-forming anaerobic rod. It causes tetanus (a spastic
paralysis caused by tetanospasmin, which blocks the release of the inhibitory neurotransmitters glycine and
gamma-aminobutyric acid [GABA]). There may be no lesion at the site of inoculation, and exudation would be
Toxocara canis (choice E), a common intestinal parasite of dogs, is a metazoan parasite that causes visceral
larva migrans. Young children are most likely to be affected, as they are most likely to ingest soil contaminated
with eggs of the parasite.
The World Health Organization identifies an alarming increase in hospital admissions worldwide attributable to a
new and unexpected serotype of influenza A virus. The biological attribute of influenza A virus, which allows the
sudden appearance of dramatically new genetic variants, is also present in a limited number of other viral
families. Which of the following viruses also possesses this biological attribute?
The correct answer is E. This case describes a pandemic of influenza A, which is caused by the ability of the
virus to undergo dramatic genetic changes of type by reassortment of its segmented RNA genome - a trait
called genetic shift. The only virus on the list that possesses a segmented genome is the rotavirus, in the
reovirus family, which possesses 10-11 segments in its genome.
Coronavirus (choice A) is not segmented and is a cause of the common cold.
HIV (choice B) is not segmented and is known for its genetic drift (minor mutational changes over time due to
an error-prone polymerase), not genetic shift.
Measles virus (choice C) is not segmented and is controlled largely by vaccination.
Rabies virus (choice D) is not segmented.
Rubella virus (choice F) is not segmented.
St. Louis encephalitis virus (choice G) is a flavivirus and is not segmented.
A 1-week-old female infant with symptoms of vomiting and anorexia has a temperature of 102° F. A bulging
fontanel is noted on physical examination. The most likely agent is
E. Streptococcus agalactiae
The correct answer is E. Vomiting, anorexia, high fever (above 100.4° F), and a bulging fontanel equals
neonatal meningitis until proven otherwise. Streptococcus agalactiae (group B strep) and Escherichia coli (not
an answer choice) are the most common causes in neonates up to 1 month of age. The next most reasonable
response would have been Listeria monocytogenes(choice B), another, though less common, cause of neonatal
Most cases of meningitis caused by Haemophilus influenzae(choice A) occur in children between 6 months and
6 years of age, 90% of which result from the capsular type b strain. It has become much less prevalent since the
H. influenzae type b conjugate vaccine has been routinely administered to infants.
Neisseria meningitidis(choice C) is the most common cause of epidemic meningitis. The two organisms most
often associated with sporadic cases are Haemophilus influenzae and Streptococcus pneumoniae (the most
common cause in adults over 30).
Staphylococcus aureus(choice D) is not a common cause of meningitis, except in patients with CSF shunts. It is
often responsible for abscesses, osteomyelitis, endocarditis, toxic shock syndrome, and food poisoning.
A 7-month-old child presents with a 4-day history of fever, deepening cough, and dyspnea. A chest x-ray shows
multiple interstitial infiltrates and hyperinflation of the lungs. Multinucleated giant cells with cytoplasmic inclusion
bodies are seen when a nasal wash is inoculated into culture. The most appropriate therapy includes
administration of which of the following drugs?
The correct answer is C. Ribavirin is an antiviral drug approved for the treatment of severe respiratory syncytial
virus infection, the most common cause of pneumonia and bronchiolitis in children under 1 year of age. It should be given by aerosol.
Acyclovir (choice A) is a guanosine analog that is useful for the treatment of primary and recurrent herpes
infections and herpes simplex virus encephalitis.
Ganciclovir (choice B) is a guanosine analog used in the treatment of cytomegalovirus retinitis and
cytomegalovirus infections in AIDS patients.
Trifluorothymidine (choice D) is a thymidine analog used topically for the treatment of recurrent epithelial
keratitis and primary keratoconjunctivitis due to herpes simplex viruses.
Zidovudine (choice E) is a thymidine analog that inhibits reverse transcriptase. It is active against human
retroviruses, including HIV-1, HIV-2, and HTLV-1.
A 6-year-old boy presents to the pediatric clinic with fever and earache. He has just finished an unsuccessful
course of amoxicillin. On physical exam, his right tympanic membrane appears injected. Which of the following
antimicrobials would be most appropriate to prescribe for this patient?
The correct answer is C. The drug of choice for otitis media in children is amoxicillin. But in refractory cases,
often due to bacterial resistance, switching to a different drug class is often effective. You must look for another
medication that is effective against common organisms responsible for pediatric otitis media, such as
Streptococcus pneumoniae (a gram-positive diplococcus) and Haemophilus influenzae (a gram-negative rod). A
second-generation cephalosporin, such as cefaclor, should cover both and is the best choice. Consequently, it
is commonly used in cases of amoxicillin-resistant otitis media. None of the other choices cover the proper
spectrum of organisms.
Amphotericin B (choice A) is an antifungal polyene. It works by binding to ergosterol in the fungal cell
membrane, creating an artificial pore. It is used to treat systemic mycoses such as Aspergillus, Blastomyces,
Candida, Coccidioides, Cryptococcus, and Histoplasma.
Bacitracin (choice B) is a topical agent used to fight infection with gram-positive organisms. It interferes with cell
Erythromycin (choice D) is a macrolide antibiotic that binds to the 23s rRNA portion of the 50s subunit of
ribosomes, inhibiting release of uncharged tRNA and stopping protein synthesis. Though effective against S.
pneumoniae, it is not particularly active against H. influenzae. Note that erythromycin may be used in
amoxicillin-resistant otitis media, but only when administered with a sulfonamide such as sulfisoxazole.
Sulfamethoxazole (choice E) is a sulfonamide. It is bacteriostatic and works by inhibiting folic acid synthesis. It
resembles p-aminobenzoic acid (PABA) structurally. When combined with trimethoprim (a dihydrofolate
reductase inhibitor) it exerts a bactericidal effect and serves as the drug combination of choice for complicated
urinary tract infections.
A previously healthy 18-month-old girl is brought to the office with 2 days of irritability, poor appetite, and pulling
at her left ear. She has no known allergies, and her vaccinations are up-to-date. On examination, the child's
temperature is 102.8 F. She is easily consoled by the mother and moves her neck spontaneously without
discomfort. There is a clear discharge from the nares. The left tympanic membrane is erythematous, dull, and
bulging. Which of the following virulence factors is generally absent in the strains of the causative organism that
produce otitis media, compared with those that produce epiglottitis or meningitis?
G. Polyribitol phosphate
The correct answer is G. This is most likely a case of Haemophilus influenzae otitis media. 95% of all cases of
invasive disease (epiglottitis, meningitis) due to H. influenzae are caused by type b organisms that possess a
polyribitol phosphate capsule. Otitis media is generally not caused by type b organisms.
β-Lactamase (choice A) is an important pathogenic feature of Moraxella catarrhalis, which is another
important cause of otitis media, but would not be an agent of epiglottitis or meningitis.
IgA protease (choice B) is produced by Streptococcus pneumoniae and Neisseria meningitidis. Both of these
cause meningitis, but not as commonly in this age group, and would not be the most common causes of otitis
media in this case.
Lipopolysaccharide (choice C) is present in all gram-negative bacteria and would not be a distinguishing feature
between those that cause otitis media and epiglottitis.
Nonpilus adhesins (choice D) are mediators of attachment to the epithelium and colonization of the oropharynx,
but would not be a primary difference between the agents of otitis media and epiglottitis.
Pili (choice E) also mediate attachment to the oropharynx, but would not be the major difference between the
agents of otitis media and epiglottitis.
Pneumolysin (choice F) is a cytotoxin produced by S. pneumoniae that destroys ciliated epithelial cells.
Which of the following organisms is most likely to be implicated as a cause of urethritis that persists after
antibiotic therapy for gonorrhea?
The correct answer is B.Chlamydia, Mycoplasma, and Ureaplasma are not effectively treated by penicillins and
cephalosporins, and are important causes of post-gonococcal urethritis. Chlamydial urethritis can be diagnosed
by using fluorescent antibodies to identify inclusions in epithelial cells.
Actinomyces(choice A) is a mouth commensal that rarely causes a deeper oral infection.
Mycobacteria(choice C) cause chronic granulomatous diseases such as tuberculosis and leprosy.
Nocardia(choice D) can cause necrotizing pneumonia and disseminated disease.
Rickettsia(choice E) cause typhus and Rocky Mountain spotted fever.
Which of the following is more frequently associated with Klebsiella pneumoniae than with Pseudomonas
E. Upper lobe cavitation
The correct answer is E.Klebsiella pneumoniae is a well-recognized cause of community-acquired lobar
pneumonia associated with cavitation. It is found typically in alcoholic males over 40 years of age with
underlying diabetes or obstructive lung disease. Klebsiella pneumoniae mimics Streptococcus pneumoniae as a
pulmonary pathogen except that Klebsiella has a greater tendency to progress to lung abscess and empyema.
Pseudomonas aeruginosa is usually associated with patients on ventilators, particularly in intensive care units.
Immunocompetent patients usually have bilateral bronchopneumonia without cavitary lesions.
Artificial ventilation (choice A) is classically associated with P. aeruginosa infection. The organism thrives in a
wet environment such as respirators, cleaning solutions, disinfectants, sinks, vegetables, flowers, endoscopes,
and physiotherapy pools.
P. aeruginosa is a very important pathogen. Mucoid strains of this organism infect the airways in patients with
cystic fibrosis (choice B), leading to acute exacerbations and chronic progression of lung damage.
Both organisms cause disease in association with diabetes (choice C). Klebsiella pneumoniae produces
pulmonary disease and P. aeruginosa causes necrotic skin ulcers in diabetics.
Green-colored sputum (choice D) is associated with P. aeruginosa, since more than half of the clinical isolates
produce the blue-green pigment pyocyanin, which is helpful in identifying the organism.
A 65-year-old man develops a vesicular rash localized to a narrow circumferential band on one side of his chest.
The rash is very painful, and the vesicles are confluent with some ulceration. No other significant findings are
demonstrated on physical examination. Which of the following diagnoses is most likely to be correct?
The correct answer is E. This is shingles, the recurrent form of herpes zoster infection, which is usually (except
in the case of immunosuppressed patients) confined to a single dermatome. Isolated vesicles may be seen
outside the dermatome. The primary herpes zoster infection precedes the development of shingles by years or
decades; the prevalence of shingles rises steadily with age, to the point that 1% of people older than 80 years
have the condition. Shingles lesions are infections and should be considered an infectious hazard in the
hospital setting. Acyclovir can ameliorate the condition.
Varicella, or chicken pox (choice A), is the primary form of herpes zoster infection and affects face and trunk
Herpes simplex I (choice B) affects oral and perioral sites.
Herpes simplex II (choice C) primarily affects genital sites.
Measles (choice D) causes a blotchy rash, rather than a dermatomal one.
A 35-year-old woman presents to her gynecologist because of vaginal itchiness and discharge. Pelvic examination demonstrates abundant white, curdy material in the vagina. Microscopic examination of the material demonstrates fungal hyphae and yeast forms.
Which of the following systemic diseases can predispose for this condition?
B. Diabetes mellitus
The correct answer is B. The patient has vulvovaginitis secondary to Candida infection. Predisposing factors include a high vaginal pH, diabetes, and use of antibiotics. The increased vulnerability in diabetes may reflect increased glucose concentrations in vaginal secretions and relative immunosuppression.
Crohn's disease (choice A) can predispose for fistulas involving the vagina, rather than vulvovaginitis.
Dissemination of Neisseria gonorrhoeae(choice C) can cause septic arthritis.
Neither rheumatoid arthritis (choice D) nor systemic lupus erythematosus (choice E) are specifically associated with Candida vulvovaginitis.
A 23-year-old woman presents to the emergency room with pelvic pain. A Gram's stain of her cervical discharge reveals multiple polymorphonuclear leukocytes, but none contain gram-negative diplococci.
Which of the following statements best describes the two organisms that most commonly cause this disorder?
C. Both induce endocytosis by epithelial cells
The correct answer is C. In young women, the most likely causes of cervicitis and pelvic inflammatory disease (PID) are Neisseria gonorrhoeae and Chlamydia trachomatis. Gram's staining alone may not be able to distinguish between the two in women, so culture is warranted. Both organisms induce endocytosis by epithelial cells.
Neisseria gonorrhoeae is a gram-negative diplococcus. It is endocytosed by mucus-secreting epithelia, and is exocytosed into subepithelial tissues leading to necrotizing acute inflammation and destruction of ciliated and non-ciliated cells. Through direct extension, it can lead to PID and infertility in women. It can be detected by visualizing the organism in polymorphonuclear leukocytes (PMNs) in Gram's stained clinical material more easily in men than in women. Therefore, culture onto Thayer-Martin agar is usually required for diagnosis in women.
Since the organism can be cultured, it is not an obligate intracellular parasite (compare with choice D). The organism can be killed by PMNs, antibody, and complement, however, many of its outer membrane proteins undergo antigenic or phase variation or cause the production of blocking antibody that interferes with bactericidal activity. Immunity to the organism does not develop (compare with choices A and B). Neisseria gonorrhoeae is resistant to penicillin, a β-lactam antibiotic (compare to choice E), because of a plasmid-mediated penicillinase and a chromosomally-mediated mutation that decreases the permeability of the outer membrane and decreases the affinity of penicillin binding proteins. It can be treated with ceftriaxone, a cephalosporin, which resists the penicillinase, or with quinolones and azithromycin.
Chlamydia trachomatis is an obligate intracellular parasite (choice D) that induces endocytosis and resides in the phagosome of infected epithelial cells. It possesses a gram-negative envelope but lacks peptidoglycan, and hence is intrinsically resistant to all antibiotics that inhibit peptidoglycan synthesis, such as the beta-lactam antibiotics (compare with choice E). Antibodies are ineffective at controlling infection. Cell mediated immunity
(choice A) is probably the major means of controlling this infection since a deficiency in cell mediated immunity both increases susceptibility and severity in animal models. Chronic and clinically latent infections cause serious morbidity, including pelvic inflammatory disease, fallopian tube dysfunction and blindness. Chlamydia infection can be treated with macrolides, quinolones and tetracyclines.
While playing in the park, an 8-year-old girl is bitten in the leg by a neighbor's cat. She presents the next day with fever and bone pain localized to her right calf. X-ray reveals a lytic lesion of the right tibia. Results of the bone culture are pending. You expect the infecting organism to be
D. Pasteurella multocida
The correct answer is D. This patient has osteomyelitis due to a cat bite that penetrated the periosteum. Whenever you see dog or cat bites in a question stem, consider Pasteurella multocida as a primary cause of wound infection. This organism is a short, encapsulated, gram-negative rod demonstrating bipolar staining. Rapidly arising cellulitis is particularly indicative of this organism.
Brucella melitensis(choice A) is a small, acapsular, gram-negative rod that causes brucellosis (undulant fever) and is associated with contact with goats or sheep. Brucella abortus and Brucella suis are variants associated with contact with cows and pigs, respectively. The organisms enter the body through the skin or through contaminated dairy products, such as unpasteurized imported goat's milk or cheeses.
Eikenella corrodens(choice B) is commonly found in human bites. It is a gram-negative rod that is part of the normal flora of the human mouth.
Francisella tularensis(choice C) is a small, pleomorphic, gram-negative rod that causes tularemia (rabbit fever).
It occurs most commonly in rural areas. In the USA, rabbits are the main reservoir for this organism, which is transmitted to humans by the Dermacentor tick or by contact with infectious animal tissues.
Yersinia pestis(choice E) is responsible for bubonic plague, which has been known to occur in the western USA. Its main reservoir is the prairie dog, and its vector is the rat flea.
A 6-week-old infant is brought to the emergency room with a 10-day history of coughing and choking spells. The white blood cell count is elevated with 80% lymphocytes. The child is gasping for breath, experiencing paroxysms of coughing, and vomits twice in the examination room. Encapsulated, gram-negative rods grow out on Bordet-Gengou media.
Which of the following microorganisms is the likely cause of these symptoms?
A. Bordetella pertussis
The correct answer is A. The child has whooping cough, or more accurately if one goes by the chronology of the symptoms, "coughing whoop," as the patient is subject to a series of coughing episodes that are followed by a rapid inspiration of air; the "whoop" is caused by the rapid passage of air through a narrowed airway. The infection of the respiratory tract by Bordetella pertussis causes a hyperreactivity of the respiratory apparatus; even the slightest stimulus will trigger a coughing episode. The offending pathogen excretes adenylate cyclase and also produces an exotoxin that inactivates the inhibitory subunit of the G-protein complex, thus activating adenylate cyclase within the cells of the respiratory system. Bordet-Gengou agar is the blood-enriched (15% vs. 5% for normal blood agar) medium used for the laboratory isolation of this pathogen. Fluorescent antibody testing is used to verify the diagnosis.
HAEMOPHILUS INFLUENZAE TYPE B (choice B) was the major cause of infant meningitis before the Hib conjugate vaccine nearly eradicated this pathogen from the United States. Nonencapsulated strains cause otitis media in children and pneumonia in adults. This organism also has a peculiar growth requirement, requiring factor X (hematin) and factor V (NAD).
KLEBSIELLA PNEUMONIAE(choice C) 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.
LEGIONELLA PNEUMOPHILA(choice D) is another fastidious, gram-negative, respiratory pathogen that may cause either a fulminating disease or a mild "walking pneumonia-like" condition (i.e., an atypical pneumonia). The organism can be cultured on a charcoal yeast extract medium, but identification is usually accomplished by immunofluorescent examination of the pulmonary specimen.
MYCOPLASMA PNEUMONIAE(choice E) is the most common cause of primary atypical pneumonia. The disease is considered "atypical" because the patients have a very mild disease with low-grade fever, little in the way of constitutional signs, and a non-productive cough. These organisms are fastidious and are difficult to grow in the laboratory. Diagnosis is by immunofluorescent staining or by demonstration of cold hemagglutinins and strep MG agglutinins in the serum of the patient.
A patient is admitted to a psychiatric hospital after having been picked up by the police for making inappropriate sexual advances. A detailed psychiatric interview demonstrates deficits in memory, insight, judgement, personal appearance, and social behavior. The patient is witnessed experiencing a possible epileptic seizure. Over a period of several years, motor findings also develop, including relaxed, but expressionless facies, tremor, dysarthria, and pupillary abnormalities.
Which of the following tests performed on his cerebrospinal fluid would most likely be diagnostic?
The correct answer is B. The patient has neurosyphilis, specifically general paresis, a term that means "general paralysis of the insane." In this late sequela of syphilitic infection which occurs 5 to 20 years after infection, patients develop mental deterioration, which precedes motor system deterioration, leading eventually to "general paralysis" with mutism and incontinence. The abnormalities may be conveniently recalled using paresis as a mnemonic for personality, affect, hyperactive reflexes, Argyll Robertson pupils in the eyes, defects in the sensorium, intellectual decline and deficient speech. Specific anti-treponemal tests such as FTA-ABS are usually positive on both serum and cerebrospinal fluid.
The cerebrospinal (CSF) glucose (choice A) in neurosyphilis is usually normal.
Gram's stain (choice C) of CSF will not demonstrate spirochetes in neurosyphilis.
The CSF lymphocyte count (choice D) is typically elevated in neurosyphilis, but this is a non-specific finding.
The CSF neutrophil count (choice E) is usually normal in neurosyphilis.
The electron transport system of Neisseria is located on which of the following structures?
A. Cytoplasmic membrane
The correct answer is A. Unlike eukaryotic cells in which the electron transport system is located on
mitochondria, the electron transport system of bacteria is located on the cytoplasmic (plasma) membrane.
Mesosomes (choice B) and polyribosome DNA aggregates (choice E) are also found in bacterial cells and
function in cell division and protein synthesis, respectively.
Bacteria do not have mitochondria (choice C) or membrane-bounded nuclei (choice D).
A Pap smear from a 30-year-old woman demonstrates protozoal parasites. These organisms are likely to be
E. Trichomonas vaginalis
The correct answer is E.Trichomonas vaginalis (note "vagina" in the name) is the only organism listed to
primarily affect the genital tract rather than the intestinal tract. The vaginitis it causes is characterized by a
frothy, yellow discharge. The organism can be identified in Pap smears, or, more reliably, by special culture
techniques that are becoming more widely available. Infection in men is usually asymptomatic, but prostatitis
and urethritis can also occur. Treatment with metronidazole is indicated for both the patient and the sexual
The other organisms listed (choices A, B, C, and D) cause gastrointestinal disease.
A previously healthy European child who had been diving into a lake develops meningoencephalitis. The disease
is rapidly fatal, despite aggressive medical therapy. Which of the following organisms is most likely to be
responsible for this child's illness?
E. Naegleria fowleri
The correct answer is E. Two free-living amoeba have a specific propensity for causing meningoencephalitis:
Naegleria fowleri and Acanthamoeba sp. The former is seen in the clinical setting described in the question
stem, and the infection appears to develop when amoebae in lake water are forced across the cribriform plate
in the nose during swimming and particularly diving. Since the olfactory bulbs are actually part of the brain,
crossing the cribriform plate allows the amoeba access to the meninges and brain. Diagnosis can be difficult
because the causative organism may resemble human cells when seen in cerebrospinal fluid. Unfortunately,
treatment is also difficult because the organism is not very sensitive to presently available anti-protozoan drugs.
Acanthamoeba sp. (choice A) can also cause meningoencephalitis, but the usual setting is in
Balantidium coli(choice B) is a ciliated organism that occasionally causes dysentery resembling that caused by
Entamoeba histolytica(choice C) causes amoebic dysentery and liver abscesses.
Giardia lamblia(choice D) is a flagellated organism that infects the small intestine, causing diarrhea.
Which of the following microorganisms is an obligate aerobe?
D. Mycobacterium tuberculosis
The correct answer is D.Mycobacterium tuberculosis is an important obligate aerobe to remember for USMLE
Step 1. (Other obligate aerobes include all other Mycobacteria, Bordetella pertussis, and Francisella
tularensis.). This organism can obtain energy only from respiration and must use oxygen as the terminal
electron acceptor. Primary infection occurs in the upper lobes of the lung.
Bacteroides fragilis(choice A) is an obligate anaerobic gram-negative bacillus that constitutes the primary
organism found in the colon. It is the most common cause of anaerobic infections, including abdominal
abscesses and peritonitis.
Clostridium perfringens(choice B), along with all the other Clostridium species, is an obligate anaerobic,
spore-forming, gram-positive bacillus. C. perfringens is an important cause of infection following trauma and
surgery and can cause gas gangrene (which can be life-threatening if not treated). C. perfringens can also
cause food poisoning.
E. coli(choice C) and P. aeruginosa(choice E) are both facultative organisms, meaning that they can grow both
in the presence or absence of oxygen. E. coli can ferment under anaerobic conditions or respire using oxygen
as the terminal electron acceptor. P. aeruginosa cannot ferment. It respires only, using nitrate as the terminal
acceptor under anaerobic conditions and oxygen as the terminal electron acceptor under aerobic conditions
A sexually active 18-year-old woman presents with a fever of 102 F for the past 24 hours and lower abdominal pain and anorexia for the past 5 days. On physical examination, there is generalized tenderness of the abdomen, and the cervix is erythematous with motion tenderness. There is no rash nor any lesions on the external genitalia.A smear of the odorless cervical discharge contains sloughed epithelial cells and scant neutrophils.
Which of thefollowing would likely be found in the exudate?
B. Iodine-staining intraepithelial inclusion bodies
The correct answer is B. The presentation is typical for pelvic inflammatory disease (PID). Chlamydia
trachomata (serotypes D-K) is the most common bacterial cause of sexually transmitted disease (STD) in this country and is the most likely agent on the list to produce the symptoms described. It is an ATP-defective organism that must therefore live intracellularly in the human host and can be visualized inside epithelial cells with iodine, Giemsa, or fluorescent-antibody stains. The remainder of the answer choices refer to other agents that could be found in the female genital tract, either by sexual transmission or by contamination with fecal flora, but they are not the best choices.
A naked, icosahedral double-stranded circular DNA virus (choice A) refers to human papilloma virus, which is the most common cause of STDs in the U.S., but presents with anogenital warts.
Intraneutrophilic gram-negative diplococci (choice C) refers to Neisseria gonorrhoeae, which would be expected to present with dysuria and neutrophilic exudate.
Intranuclear "owl's eye" inclusion bodies (choice D) refers to cytomegalovirus, a common STD in the United States, but not a common agent of PID. Most cases in average adults are manifested by mononucleosis-like symptoms.
Lactose-fermenting gram-negative bacilli (choice E) would be consistent with Escherichia coli. Although this organism is the most common cause of urinary tract infections in women in the United States, it would not be expected to cause PID.
Pear-shaped flagellated protozoa (choice F) refers to the protozoan parasite Trichomonas vaginalis, the only protozoan STD in the world. Infection would be characterized by a malodorous, cheesy exudate, and there would be more erythema of the external genitalia than of the cervix.
Pleomorphic, gram-negative rods (choice G) are consistent with Hemophilus ducreyi, which causes chancroid, and presents in a different manner.
Spirochetes on dark-field microscopy (choice H) refers to Treponema pallidum, the causative agent of syphilis, which would produce rash and/or chancre, depending on the stage of the infection.
Which of the following types of viruses would be most likely to undergo an abrupt, major antigenic shift permitting
reinfection of previously exposed individuals?
The correct answer is D. The phrase "antigenic shift" should tip you off to look for "influenza viruses" or
"orthomyxoviruses" in the answers. The orthomyxoviruses include influenza viruses A, B, and C. These viruses
are successful at reinfecting previously exposed individuals because their genome consists of about eight (the
viral packaging process is a little vague and sometimes entraps more pieces) separate segments of RNA. Minor
antigenic changes are frequent and are called antigenic drift. Additionally, major changes, called antigenic shift
rather than antigenic drift, rarely occur, possibly as a result of double infection of cells by human and animal
influenza viruses with resulting accidental exchange of whole RNA segments. Such dramatic antigenic shifts
have occurred four times for influenza A since 1933. The other viruses listed in the answers do not undergo
such dramatic shifts.
A young mother takes her baby to the pediatrician for the first time. The doctor notices the infant's teeth have
yellow discolorations. The antibiotic this mother most likely took during pregnancy
A. Inhibits aminoacyl-tRNA binding
The correct answer is A. This question relates to a USMLE favorite side effect—the teeth mottling that
occurs when a child is exposed to tetracycline in utero. You should remember that tetracycline is contraindicated in pregnancy and early childhood. Tetracycline is a bacteriostatic drug that binds to the 30s
subunit of ribosomes, preventing aminoacyl-tRNA from binding with complementary mRNA. This inhibits peptide
bond synthesis. Resistance is plasmid mediated.
Inhibition of peptidyl transferase (choice B) occurs with chloramphenicol, a broad-spectrum bacteriostatic agent
that binds to the 50s subunit of ribosomes. Resistance is plasmid mediated. It has high toxicity (GI disturbances,
aplastic anemia, and gray baby syndrome), so it is used mainly in severe infections or as a topical agent.
Interference with cell wall synthesis (choice C) occurs with penicillins and cephalosporins, the beta-lactam
antibiotics. Resistance to these drugs appears in organisms that have developed beta-lactamases
(penicillinases), enzymes that destroy the beta-lactam ring of these medications. The wider spectrum ampicillin,
amoxicillin, ticarcillin, and carbenicillin are particularly penicillinase susceptible.
Large, cyclic, lactone-ring structures (choice D) describe the macrolides: erythromycin, azithromycin, and
clarithromycin. They inhibit bacterial protein synthesis by reacting with the 50s ribosomal subunit and
preventing the release of the uncharged tRNA. Resistance is plasmid mediated. Common side effects include GI
irritation, skin rashes, and eosinophilia. Erythromycin is a popular choice for patients with penicillin
hypersensitivity. It is a cytochrome p450 inhibitor and therefore must be used with caution in patients taking
A 32-year-old man from Yuma, Arizona presents to a hospital with complaints of a 4-day history of fever,
myalgias, and cough. He owns an automobile repair shop and had recently cleaned his garage, which was
infested by mice. On physical examination, he is tachypneic and must use accessory respiratory muscles to
breathe. Shortly following admission, he is intubated and diagnosed with adult respiratory distress syndrome.
What underlying infection should be considered?
The correct answer is C. Hantavirus pulmonary syndrome, first reported in 1993, consists of fever, myalgia, and
rapid development of respiratory failure leading to death. The initial cases were mostly among the Navajo
Indians; however, cases have been documented in many states and in Canada. It is most common in New
Mexico, Arizona, Colorado, and Utah. The causative agent is a Hantavirus belonging to the family Bunyaviridae.
The virus is now called Sin Nombre (meaning "without a name") virus. Infection is thought to be through
inhalation of aerosolized secretions from the common deer mouse. The disease typically begins with a
non-specific prodrome (fever, generalized myalgia, and gastrointestinal disturbances) followed 4 to 5 days later
by respiratory symptoms (cough, dyspnea, and tachypnea). This progresses rapidly to an adult respiratory
distress syndrome and, in many cases, death.
California encephalitis (choice A) is caused by an arbovirus transmitted by infected mosquitoes. Most cases are
in children less than 10 years of age. Signs and symptoms include fever, headache, photophobia, nausea, and
vomiting. Myalgias and arthralgias are typically present. Seizure activity is most frequently observed in infants
and young children. Most cases are in the midwestern U.S.
Cytomegalovirus (choice B) is a herpesvirus producing a variety of infections. Primary CMV infection is usually
asymptomatic in immunocompetent patients, but can cause a heterophile-negative mononucleosis syndrome.
Perinatal infections can occur in utero, intrapartum, or postpartum and can produce congenital malformations.
CMV can be transmitted by granulocytes in blood transfusions. In patients with AIDS or other
immunosuppressive states, CMV can produce severe disease, including retinitis, pneumonia, encephalitis,
adrenalitis, and gastrointestinal tract ulcerations.
Poliovirus (choice D) is a member of the Picornavirus family. It remains epidemic in parts of Asia and Africa even
though the wild-type has been eliminated from the western hemisphere. The virus affects cranial nerve nuclei
and anterior horn motor neurons of the spinal cord, producing a flaccid paralysis, which is usually asymmetric.
Vaccine-related polio can occur with the live virus vaccine.
Rabies (choice E) is a Rhabdovirus that produces encephalitis and/or myelitis. It should be considered in
persons who have recently traveled outside of the U.S. The most common sources of exposure are dogs, cats,
skunks, foxes, raccoons (in Florida and Connecticut), wolves, and bats. The infection is acquired by the bite of
a rabid animal or by inhalation. Rabies also has been reported to occur in patients after corneal transplantation.
The virus spreads along peripheral nerves to the central nervous system. Symptoms include hydrophobia and
A 27-year-old woman presents to the emergency department complaining of 10-12 episodes of nonbloody diarrhea per day for the past 2 days, along with severe abdominal cramps, nausea, vomiting, and a low-grade fever. She states that she just returned from a vacation to Mexico. While in Mexico, she did not drink any of the local water and ate only cooked foods and a few fresh salads. If fecal leukocytes are present, the patient should most likely be empirically treated with
The correct answer is B. When individuals travel from one country to another with marked differences in climate and sanitation standards, the risk for development of traveler's diarrhea is dramatically increased. This type of diarrhea is most likely to develop within 2-10 days after ingesting local water and/or eating fresh fruits and vegetables (such as a fresh salad) that could have been "washed off" with the local water. Traveler's diarrhea is often accompanied by the appearance of approximately 10 or more episodes of diarrhea per day, as well as severe abdominal cramps, nausea, vomiting, and a mild fever. Depending on the causative bacteria, blood and/or fecal leukocytes may be present. Most cases of traveler's diarrhea are caused by enterotoxigenic Escherichia coli, Shigella species, and Campylobacter jejuni. The most commonly used agents to treat traveler's diarrhea are the fluoroquinolones, such as ciprofloxacin, ofloxacin, and norfloxacin.
Trimethoprim/sulfamethoxazole is most commonly used in treatment of children. None of the other agents are indicated for the treatment of traveler's diarrhea.
Acyclovir (choice A) is an antiviral agent indicated for the treatment of herpes virus infections.
Mebendazole (choice C) is a broad-spectrum anthelminthic indicated for the treatment of "worm" infections, such as Enterobius vermicularis, Trichuris trichiura, and Ascaris lumbricoides.
Quinine (choice D) is an antimalarial agent indicated for the treatment of malaria and severe leg cramps.
Tetracycline (choice E) is an antibacterial agent with a limited antibacterial spectrum; it is primarily used in the treatment of acne vulgaris and gonorrhea infections.
A 7- year-old girl develops behavioral changes, and her performance in school begins to deteriorate. Several months later she develops a seizure disorder, ataxia, and focal neurologic symptoms. She is eventually quadriparetic, spastic, and unresponsive. Death occurs within a year. This patient may have had which of the following viral diseases at 1 year of age?
The correct answer is C. The child has subacute sclerosing panencephalitis (SSPE), which is fortunately a very rare, late complication of measles infection at an early age. The existence of this complication is part of the rationale to immunize children to measles at an early age. It is thought that very young children's immune and neurologic systems may permit the virus to become established in the brain. The exact mechanism of injury is poorly understood, but the brain shows encephalitis involving both gray and white matter. 40% of cases die within 1 year, and it is thought that the disease is probably always eventually fatal.
None of the other diseases progresses to SSPE.