A 16-year-old, previously healthy girl comes to the office due to vaginal discharge that began 5 days ago.  The discharge is grayish-white and fishy smelling.  The patient is sexually active with a male partner and occasionally uses condoms.  Temperature is 36.7 C (98.1 F), blood pressure is 106/52 mm Hg, and pulse is 78/min.  Examination shows a small amount of grayish discharge in the vaginal vault.  The cervix appears normal.  Which of the following would most likely be seen on wet mount microscopy?

A 38-year-old woman comes to the emergency department due to abdominal discomfort and diarrhea.  Her symptoms began after she returned yesterday from a Caribbean cruise.  She consumed bottled water but ate raw, local seafood when visiting one of the islands.  The cruise line has reported no other passengers with abdominal illness.  On examination, the abdomen is soft with mild, generalized tenderness and hyperactive bowel sounds.  A comma-shaped, oxidase-positive, gram-negative rod naturally found in salt water is implicated in her illness.  Stool microscopy is most likely to demonstrate which of the following findings?

A 42-year-old man comes to the office due to 2 weeks of progressive fever, chills, abdominal discomfort, and loose stools, which began during a trip to Southeast Asia.  He received no pretravel vaccinations and did not strictly follow food and water safety precautions during the trip.  The patient has no chronic medical conditions; however, he had an episode of Salmonella Enteritidis gastroenteritis last year, and it resolved with symptomatic care.  Temperature is 39.4 C (102.9 F), blood pressure is 110/64 mm Hg, pulse is 62/min, and respirations are 18/min.  Physical examination shows a faint macular rash on the trunk.  There is mild tenderness to palpation in the lower abdominal quadrants with no guarding or rebound tenderness.  Hepatosplenomegaly is present.  Blood culture grows gram-negative, non–lactose-fermenting rods.  Which of the following pathogenic mechanisms best explains this patient’s prolonged and severe disease course compared to his prior episode of bacterial gastroenteritis?

A 34-year-old woman comes to the office due to fever, dysuria, and flank pain.  Temperature is 37.8 C (100 F), blood pressure is 122/78 mm Hg, and pulse is 84/min.  Physical examination elicits suprapubic and costovertebral angle tenderness.  Urine dipstick analysis reveals positive leukocyte esterase and nitrite.  A subsequent urine culture grows colonies of motile gram-negative rods demonstrating a green metallic sheen on eosin methylene blue (EMB) agar and hemolysis on blood agar.  Which of the following virulence factors is most important for the development of this patient’s condition?

A 52-year-old man is brought to the emergency department due to worsening right leg pain, fever, and confusion.  The patient injured his leg while operating a motorized watercraft on the ocean near Florida 2 days ago.  Temperature is 38.9 C (102 F), blood pressure is 90/50 mm Hg, and pulse is 120/min.  The patient is lethargic and diaphoretic.  Physical examination reveals a small laceration on the dorsum of the right foot with surrounding edema, erythema, and several hemorrhagic bullae.  Leukocyte count and serum lactic acid levels are elevated.  Intravenous fluids and empiric antibiotics are administered, and surgical debridement of the wound is performed.  Blood and wound cultures yield curved gram-negative rods.  Which of the following is the greatest risk factor for this patient’s infection?

A 23-year-old woman comes to the emergency department due to 2 hours of fever, chills, headache, and myalgias.  Earlier in the day, the patient saw her primary care physician for a genital ulcer.  She underwent testing for sexually transmitted infections and was found to have a positive VDRL and fluorescent treponemal antibody absorption test.  The remainder of her testing was negative.  The patient received an intramuscular injection of benzathine penicillin G, and several hours later her current symptoms began.  Temperature is 38 C (100.4 F), blood pressure is 120/70 mm Hg, and pulse is 92/min and regular.  Lung sounds are normal.  There is an indurated, nontender vulval ulcer and several enlarged inguinal lymph nodes.  No other skin rash is present.  Which of the following is the most likely cause of this patient’s current symptoms?

A 23-year-old woman is evaluated due to 10 days of nonproductive cough, low-grade fever, headache, and malaise.  The patient has no other medical problems and takes no medications.  Lung examination reveals scattered rales.  Chest x-ray reveals bilateral patchy areas of consolidation.  She has mild anemia and an elevated serum lactate dehydrogenase level.  The patient is treated for presumed Mycoplasma pneumonia with azithromycin.  Two months later, her symptoms and the anemia have resolved.  Which of the following best explains the resolution of this patient’s anemia?

A 5-year-old boy is brought to the emergency department with worsening somnolence, lethargy, and decreased urine output.  The patient has no prior medical conditions but had abdominal cramps and diarrhea several days ago that later became frankly bloody.  Physical examination shows mild pallor and moist mucous membranes.  Laboratory studies reveal elevated blood urea nitrogen and serum creatinine levels.  Peripheral blood smear shows fragmented erythrocytes.  This patient’s condition is most likely related to consumption of which of the following foods?

A 21-year-old man comes to the emergency department due to shortness of breath, fatigue, myalgias, and debilitating retroorbital headaches for the past several days.  The patient is on summer break and came to work at his grandparents’ dairy farm 3 weeks ago.  He cleans out animal waste from the barn, but he does not handle hay.  His past medical history is insignificant.  He does not use tobacco or illicit drugs.  The patient’s temperature is 38.8 C (101.8 F).  Bronchial breath sounds are heard in the right lower lung.  There are no cardiac murmurs.  Abdominal examination is unremarkable.  Chest x-ray demonstrates right lower and middle lobe consolidation.  Laboratory results are as follows:

An HIV test is negative.

Which of the following is the most likely cause of this patient’s symptoms?

A 26-year-old man develops dysuria and purulent urethral discharge following unprotected sex with a new partner.  Nucleic acid amplification testing is positive for Neisseria gonorrhoeae infection.  The patient develops antibody against the bacterial pili that enhance attachment to host cells.  His symptoms resolve with adequate treatment.  His partner does not receive treatment.  Several weeks later, the patient develops N gonorrhoeae reinfection after repeat exposure to the same partner.  Which of the following is the most likely reason for the lack of long-lasting immunity against the bacteria despite antibody formation in this patient?

A previously healthy 24-year-old man is hospitalized due to 3 days of fever, chills, myalgia, and severe headache.  The symptoms started on the last day of his camping trip in the Appalachian Mountains and have rapidly progressed.  Temperature is 38.9 C (102 F), blood pressure is 100/56 mm Hg, and pulse is 120/min.  On examination, the patient is lethargic and confused.  There is no neck rigidity.  The lungs are clear on auscultation and heart sounds are normal.  The abdomen is soft and nontender.  There is a prominent erythematous maculopapular rash on the distal forearms.  No lymphadenopathy is present.  The patient should be immediately initiated on a medication that targets which of the following?

A 4-year-old boy is brought to the clinic by his mother due to decreased appetite, abdominal cramps, and diarrhea over the last 3 days.  The stools were initially watery but have now become bloody.  Temperature is 38 C (100.4 F).  Mucous membranes are dry, and the abdomen is mildly distended and diffusely tender to palpation.  Stool studies are positive for fecal leukocytes and occult blood.  Stool cultures grow non–lactose fermenting, gram-negative rods on MacConkey agar.  The bacteria ferment glucose without gas production, do not generate hydrogen sulfide when grown on triple sugar iron agar, and cannot replicate at refrigeration temperatures.  Which of the following bacterial factors is most essential for the pathogenesis of this patient’s disease?

A 3-month-old boy is brought to the emergency department due to 2 days of fever and irritation that progressed to lethargy.  Temperature is 38.6 C (101.5 F).  A lumbar puncture is performed, and the results of cerebrospinal fluid (CSF) analysis are as follows:

Gram-negative coccobacilli are isolated from the CSF.  These organisms demonstrate little growth on 5% sheep blood agar but grow well when incubated on the same media alongside colonies of Staphylococcus aureus.  Staphylococci promote the growth of these bacteria by supplementing which of the following substances?

A previously healthy 23-year-old man comes to the office due to abrupt-onset high fever, chills, headache, and weakness that developed after he spent a month working on a large ranch.  His initial symptoms were quickly followed by intensely painful swellings in the groin.  Physical examination is significant for tender and enlarged inguinal lymph nodes with overlying erythematous skin.  Blood cultures and fluid aspirated from the lymph nodes grow gram-negative coccobacilli that resemble a closed safety pin on special staining.  Which of the following activities is the most likely source of this patient’s infection?

A 46-year-old man is brought to the emergency department 30 minutes after being found intoxicated in a park.  The patient reports a 10-day history of fevers, night sweats, weight loss, and productive cough with foul-smelling sputum.  He drinks 1 or 2 pints of vodka daily and has been hospitalized multiple times for intoxication.  Temperature is 38.9 C (102 F), blood pressure is 110/65 mm Hg, pulse is 102/min, and respirations are 22/min.  Lung auscultation reveals coarse crackles over the right lower lobe.  Heart sounds are normal with no murmurs.  Laboratory results show an elevated leukocyte count.  Chest x-ray reveals a cavitary lesion with an air-fluid level in the superior segment of the right lower lobe of the lung.  Which of the following is the most likely cause of this patient’s lung findings?

A 45-year-old man comes to the office with groin pain and swelling.  About a month ago, he noticed a sore on his penis but did not seek medical attention because the ulcer was not painful and disappeared within a week.  Several days ago, he began to experience painful swelling in his inguinal region, with inflammation of the overlying skin and eventual formation of several draining ulcers.  He also has mild fever and malaise that began around the same time as his groin symptoms.  The patient is a sailor.  His other medical problems include well-controlled hypertension and hyperlipidemia.  Cell scrapings from his lesions show cytoplasmic inclusion bodies.  Which of the following is the most likely cause of this patient’s condition?

An 8-year-old boy who recently moved to the United States from Indonesia is brought to the emergency department due to fever, sore throat, and difficulty breathing.  His immunization status cannot be determined.  Physical examination shows extensive, coalescing, gray pharyngeal exudate and cervical adenopathy.  Several days after being admitted to the hospital, he dies of myocarditis and severe heart failure.  The presence of which of the following would most likely have prevented this patient’s death?

A 5-year-old child is brought to the emergency department due to difficulty breathing and a low-grade fever.  The patient’s family recently moved to the United States from rural Nepal.  Temperature is 38.1 C (100.6 F).  Physical examination shows a grayish pharyngeal exudate, enlarged cervical lymph nodes, and partial soft palate paralysis.  A sample of the pharyngeal exudate is obtained by oropharyngeal swab.  Bacteria isolated from the exudate demonstrate exotoxin production in the laboratory.  Which of the following best describes the mechanism of action of the exotoxin?

A 35-year-old woman comes to the office due to a vulvar lesion.  She has also experienced occasional headaches and memory loss recently.  The patient has had unprotected sexual intercourse with multiple partners.  Cardiovascular examination is notable for a diastolic murmur with a prominent second heart sound.  There is a painless indurated nodule on her vulva.  Cervical cultures are negative for gonorrhea, but a serum Venereal Disease Research Laboratory (VDRL) test is positive.  Chest x-ray reveals calcifications at the level of the ascending aortic arch.  Blood cultures are negative.  Lumbar puncture shows mild pleocytosis and a positive VDRL result.  HIV testing is negative.  Which of the following best describes this patient’s vulvar lesion?

A 34-year-old man comes to the office due to a painless penile ulcer, which he first noticed 3 days ago.  He had unprotected sexual intercourse with a new partner a few weeks ago.  The patient has no significant medical history and takes no medications.  Temperature is 37.1 C (98.8 F).  Examination reveals a 2-cm nontender ulcer close to the glans penis with a raised, indurated margin and a clean base.  There are no surrounding lesions or vesicles.  There are several bilateral enlarged inguinal lymph nodes, which are firm, nontender, and rubbery.  Physical examination is otherwise unremarkable.  Rapid plasma reagin and HIV testing are negative.  Infection with which of the following is the most likely cause of this patient’s symptoms?

A 6-year-old boy whose family recently immigrated to the United States is brought to the office due to sore throat and fever.  Examination shows mild tonsillar erythema with exudates.  A sample from the exudates is obtained.  On microscopic evaluation, there are clumped, gram-positive bacteria with polar granules that stain deeply with aniline dyes.  Which of the following best explains the pathogenicity of the organism most likely responsible for this patient’s condition?

A 6-year-old boy is brought to the office due to persistent cough.  His symptoms began with a mild, upper respiratory illness 2 weeks ago, but the cough has progressively worsened, and now spells of dry cough are often followed by vomiting.  The patient has no chronic medical conditions but has not received childhood vaccinations due to parental beliefs.  He recently returned from a summer camp.  On physical examination, the patient is afebrile, and lungs are clear to auscultation.  Testing of respiratory secretions shows gram-negative coccobacilli.  Which of the following is most likely involved in the pathogenesis of this patient’s symptoms?

A 21-year-old man comes to the urgent care clinic with a lesion on his penis.  He noticed a painful ulcer on the right side of his penis 4 days ago and it has progressively increased in size since then.  The patient has had no dysuria, hematuria, or penile discharge.  He is a tourist visiting New York City from Botswana.  He is HIV-positive and is sexually active.  The patient currently takes no medications.  He appears comfortable and is afebrile.  On genitourinary examination, there is a 1.5-cm ulceration on the right side of the base of the penis; the lesion is very tender and soft; has irregular, ragged borders; and is covered with grey exudate.  There is a palpable, tender, enlarged inguinal lymph node in the right groin.  The examination is otherwise unremarkable.  A scraping of the ulcer is performed.  Polymerase chain reaction testing for herpes simplex virus is negative, and darkfield microscopy reveals no organisms.  Which of the following is the most likely diagnosis?

A 24-year-old woman comes to the emergency department due to joint pain.  A week ago, the patient had a flu-like illness with fever, malaise, and fleeting joint aches in her wrists, ankles, and knees.  Over the past 2 days, the joint pain worsened, and she developed new swelling of the right knee.  The patient has no chronic medical problems and has had no similar symptoms in the past.  She does not use tobacco, alcohol, or injection drugs.  She is sexually active, uses an oral contraceptive for birth control, and her last menstrual period was a week ago.  Temperature is 38 C (100.4 F).  On examination, there is no scleral icterus, facial rash, or oral ulcers but a few scattered painless vesiculopustular lesions are present on the upper extremities.  The right knee is warm, swollen, and tender with decreased range of motion.  Both ankles and the left wrist are mildly tender to palpation but not swollen or erythematous.  Arthrocentesis of the right knee joint yields 20 mL of cloudy fluid with a leukocyte count of 50,000/mm³ (90% neutrophils).  Additional evaluation of the joint fluid is most likely to show which of the following?

A 46-year-old man comes to the emergency department due to fever.  He returned from a vacation in Southeast Asia a week ago and began having symptoms toward the end of his trip.  The patient describes continuous fever that has risen slowly to reach a high plateau and is relieved only by antipyretics.  He has also had headaches and abdominal discomfort; he initially had constipation but has had loose stools for the past several days.  Temperature is 40 C (104 F), blood pressure is 114/68 mm Hg, and pulse is 62/min.  No oropharyngeal lesions, cervical lymphadenopathy, or neck rigidity is present.  Lungs are clear on auscultation, and heart sounds are normal.  Abdominal examination shows mild distension and generalized tenderness with hepatosplenomegaly.  There is a faint, erythematous, macular skin rash over the trunk and abdomen.  Laboratory studies reveal mild, normocytic, normochromic anemia and leukopenia with left shift.  Peripheral blood smear is normal.  Which of the following is the most likely route of transmission of this patient’s infection?

An 83-year-old woman is brought to the emergency department from a nursing home due to fever and altered mental status.  The patient has a history of hypertension, diabetes, dementia, fecal incontinence, and bladder dysfunction requiring intermittent urinary catheterization.  Her temperature is 38.3 C (100.9 F), blood pressure is 100/68 mm Hg, and pulse is 114/min.  She is lethargic and unable to answer questions.  Her abdomen is soft, with suprapubic tenderness.  Lungs are clear bilaterally.  There is no cardiac murmur or skin rash.  Laboratory results are as follows:

Chest x-ray reveals normal-appearing lungs.  Blood and urine cultures are performed, and empiric antibiotics are initiated.  Urine cultures are positive for bacteria that grow readily on MacConkey agar, are lactose-fermenting, and are indole-positive.  Which of the following is the most likely organism causing this patient’s infection?

A 24-year-old previously healthy man comes to the office due to a week of low-grade fever, malaise, sore throat, and persistent nonproductive cough.  He does not use tobacco, alcohol, or illicit drugs.  The patient takes no medications and has no drug allergies.  He lives in Connecticut and has not traveled recently.  Physical examination reveals mild pharyngeal erythema, no nasal congestion, and bilateral vesicular breath sounds with no crackles or wheezes.  Chest x-ray shows patchy, interstitial infiltrates.  Sputum Gram stain shows numerous leukocytes but no organisms.  Which of the following agents is the best initial treatment for this patient?

An outbreak of diarrheal illness occurs in a community after a massive hurricane.  The affected patients experience voluminous, watery diarrhea that quickly leads to severe dehydration.  Stool examinations of these patients yield oxidase-positive, curved, highly motile, gram-negative rods.  Epidemiologic study reveals the source as a contaminated water supply.  Which of the following patient populations would be most susceptible to developing this illness?

A 26-year-old woman comes to the office due to fever, malaise, and joint pain.  A week ago, the patient had pain in the small joints of her right hand, which improved spontaneously but was quickly followed by pain in the right ankle and left wrist.  She has no prior medical problems and takes no medications.  Temperature is 38 C (100.4 F), blood pressure is 124/82 mm Hg, and pulse is 90/min.  The extremity joints are not erythematous or swollen, but tenderness is present along the tendons of the left wrist and right ankle.  Several small, nontender pustules are seen on the extremities.  There is no other rash.  Blood culture in a selective growth medium consisting of chocolate agar and multiple antibiotics yields the causative pathogen.  Which of the following strategies would most likely have prevented this patient’s current condition?

A 72-year-old man is brought to the emergency department from a skilled nursing facility due to altered mental status, skin flushing, and high fever.  He uses a wheelchair for mobility since a stroke several years ago.  Temperature is 38.9 C (102 F), blood pressure is 80/40 mm Hg, pulse is 120/min, and respirations are 26/min.  A blood sample is obtained, and culture is positive for bacterial growth within a few hours.  Multiplex PCR testing of the blood detects Escherichia coli.  Which of the following is the most likely source of bacteremia in this patient?

A 32-year-old man comes to the office for a follow-up appointment.  The patient recently returned from a 10-day trip to Honduras.  On the fifth day of his trip, he developed malaise, anorexia, and abdominal cramps, quickly followed by watery diarrhea.  The patient had 5 or 6 stools daily but no fever.  He took no medications and remained hydrated by drinking bottled water and soup.  The patient felt better after 2 days and has had no additional symptoms since.  He has no other medical conditions.  Vital signs are within normal limits and physical examination shows no abnormalities.  Which of the following factors was most likely involved in the pathogenesis of this patient’s gastrointestinal symptoms?

A 22-year-old woman participates in a research study.  She is asymptomatic and has not had a respiratory illness in the last 8 weeks.  She reports no chronic medical problems and takes no medications.  A nasopharyngeal specimen is obtained and cultured on a selective medium containing vancomycin, colistin, nystatin, and trimethoprim.  The organism most likely to be isolated is typically associated with which of the following conditions?

A 24-year-old man comes to the emergency department with leg swelling.  His dog, which is fully vaccinated and has lived with him for the past year, bit him on the left leg last night, leading to a break in the skin and bleeding.  The patient irrigated the wound with bottled water and soap and managed his pain with acetaminophen.  This morning, the wound area became warm, red, and swollen.  On examination, temperature is 36.4 C (97.5 F).  There is an open wound on the left leg, with surrounding erythema, warmth, and tenderness.  No wound drainage or crepitus is present.  The distal pulses are palpable.  Wound cultures grow gram-negative coccobacilli; the culture has a mouse-like odor.  Which of the following organisms is most likely responsible for this patient’s current presentation?

A 10-week-old boy is brought to the office for a follow-up visit.  As part of his routine immunization schedule, the patient received the first dose of the Haemophilus influenzae type b (Hib) conjugate vaccine when he was 8 weeks old.  However, following vaccine administration, he developed anaphylaxis, requiring epinephrine administration and hospitalization.  As a result, the decision was made not to administer additional doses of the Hib vaccine.  The patient was born full-term by uncomplicated vaginal delivery and has otherwise been healthy.  Physical examination is unremarkable.  His older brother has completed the full vaccination course.  This patient is at highest risk of which of the following infections compared to his brother?

A 26-year-old man comes to the office due to a 3-day-history of dysuria and urethral discharge.  The symptoms developed about 2 weeks after he had unprotected sexual intercourse with a new partner.  His temperature is 37.1 C (98.8 F).  On physical examination, a mucoid discharge is expressed with gentle milking of the penis.  Gram stain of the discharge reveals numerous neutrophils with intracellular diplococci.  A sample of the discharge is placed on an antibiotic-containing medium, and bacterial colonies are cultured.  Which of the following terms best describes the medium?

A 29-year-old man is hospitalized due to a flulike illness.  His symptoms abruptly began 3 days ago with fever, rigors, malaise, headache, and myalgia.  The patient has no prior health conditions and is usually very active, participating in multiple outdoor sports.  Temperature is 38.9 C (102 F), blood pressure is 124/84 mm Hg, and pulse is 110/min.  Physical examination shows an ill-appearing man with bilateral conjunctival suffusion.  Lung and heart sounds are normal, but abdominal examination reveals mild hepatosplenomegaly.  Generalized lymphadenopathy is also present.  A test for influenza antigen is negative, and blood cultures in routine growth media are negative.  The patient subsequently develops progressive jaundice and renal failure.  Further testing detects an infection with a highly motile, corkscrew-shaped pathogen.  This patient most likely acquired the infection via which of the following?

A 59-year-old woman is brought to the emergency department with fever, skin flushing, and an altered level of consciousness.  Blood pressure is 50/20 mm Hg and pulse is 120/min despite receiving intravenous fluids.  Blood cultures are positive for Escherichia coli.  Which of the following bacterial components is most directly responsible for this patient’s current condition?