A 63-year-old man is brought to the emergency department due to high fever, confusion, headache, watery diarrhea, and cough.  The patient smokes 2 packs of cigarettes daily and has a history of chronic bronchitis.  Temperature is 40.1 C (104.2 F), blood pressure is 100/70 mm Hg, pulse is 91/min, and respirations are 28/min.  Sputum Gram staining reveals numerous neutrophils but no bacteria.  Which of the following is the most likely cause of this patient’s current condition?

A 3-year-old boy who recently immigrated to the United States is brought to the emergency department with fever, malaise, and a painful, swollen right knee.  He is hypotensive and tachycardic.  Medical history is significant for a recent episode of acute otitis media and several vaccinations that are not up to date.  Arthrocentesis of the right knee shows cloudy synovial fluid.  Gram stain of the aspirate reveals pleomorphic, gram-negative coccobacilli.  Cultures performed on a blood agar plate supplemented with a disk containing hematin and nicotinamide adenine dinucleotide (NAD⁺) grow colonies only near the disk.  The organism responsible for this patient’s condition most likely produces which of the following virulence factors?

A 16-year-old girl is brought to the emergency department after she became unresponsive.  Temperature is 39.7 C (103.5 F), blood pressure is 70/40 mm Hg, and pulse is 130/min and thready.  There is a diffuse petechial and ecchymotic skin rash.  Laboratory testing reveals leukocytosis with left shift and evidence of disseminated intravascular coagulation and multiorgan failure.  Despite aggressive interventions, the patient dies several hours after admission.  Autopsy reveals hemorrhagic necrosis of many internal organs, including the bilateral adrenal glands.  Which of the following microbial components is directly responsible for the severity of disease in this patient?

Health authorities are investigating an outbreak of respiratory infections among a group of military recruits.  Fifteen recruits reported persistent cough, low-grade fever, and malaise.  Apart from the low-grade fever, physical examination was largely unremarkable.  Chest x-rays were all notable for pulmonary infiltrates that appeared more severe than what would have been expected based on assessment of the patients’ clinical status.  Sputum specimens were obtained, and the causative organism required a complex acellular medium enriched with cholesterol to grow.  Which of the following organisms is the most likely cause of the outbreak?

A 68-year-old woman comes to the emergency department due to cough and shortness of breath.  She has a history of chronic obstructive pulmonary disease and often coughs up clear sputum in the morning.  Over the past 4 days, the cough has worsened significantly, and the patient now is producing thick, greenish sputum.  She also feels winded when walking short distances or doing light housework.  She is a former smoker with a 40-pack-year history.  Temperature is 37.2 C (99 F), blood pressure is 130/70 mm Hg, pulse is 90/min, and respirations are 22/min.  The patient is breathing with pursed lips.  The expiration phase is prolonged.  Vesicular breath sounds with diffuse wheezes are heard on lung auscultation.  Chest x-ray shows hyperinflated lungs with a flattened diaphragm.  There are no alveolar opacities or pleural effusions.  Respiratory viral panel results are negative.  Infection with which of the following pathogens most likely triggered this patient’s worsening symptoms?

A previously healthy 43-year-old man comes to the office during the early fall due to a 4-week history of cough.  He initially had a “bad cold” that lasted about 10 days.  The sneezing, runny nose, and muscle aches that accompanied the cold have improved, but the cough has persisted and worsened.  The patient has “bursts of coughing” for several minutes as he feels unable to clear the mucus.  On at least 5 occasions, these attacks were so severe that he vomited afterward.  The patient is a children’s karate teacher.  He received all of his childhood vaccinations but has not seen a physician in many years.  There is no history of recent travel.  Vital signs and physical examination are normal.  Chest x-ray is unrevealing.  Which of the following is the most likely cause of this patient’s symptoms?

A 3-year-old boy is brought to the office due to a 1-day history of fever and irritability.  The mother states that the boy has been tugging at his right ear.  The patient has had 2 previous episodes of acute otitis media.  Temperature is 38.1 C (100.6 F).  Otoscopic examination shows a perforated right tympanic membrane with erythema and purulent exudate.  Cultures from the exudate yield small, oxidase-positive, gram-negative coccobacilli that grow on factor X– and factor V–supplemented media, consistent with Haemophilus influenzae.  The patient’s immunizations are up to date.  Which of the following best explains this patient’s susceptibility to the pathogen causing his current infection?

Several residents of a long-term care facility develop febrile illness within a short period of time.  Symptoms include fever, nonproductive cough, myalgia, rigors, dyspnea, and diarrhea.  Organisms found in induced sputum samples stain poorly with Gram stain but are visualized easily using silver stain.  Urine testing for bacterial antigens is positive in most of the affected individuals.  Which of the following is the most likely source of this outbreak?

A novel antimicrobial agent that inhibits bacterial glycosyltransferase enzyme is developed.  Bacteria grown in a medium containing this antibiotic become spherical, cease to grow, and subsequently lyse.  So that the activity of this antibiotic against various bacteria can be determined, disks enriched with the drug are placed on agar plates containing bacterial colonies.  The plates are incubated alongside control plates with the same organisms but no antimicrobial disks.  The control plates all show organism growth.  In the plates with the diffusion disks, resistance to the drug is determined by measuring the zone of complete growth inhibition around the disk.  Which of the following bacteria is most likely to be resistant to this drug?

A 56-year-old woman is brought to the emergency department due to a 2-day history of high fever, headache, mild confusion, and dry cough.  She also has mild abdominal discomfort and watery diarrhea.  The patient recently returned from a cruise to Hawaii.  Her other medical problems include hypertension and hyperlipidemia.  She has smoked 1 pack of cigarettes daily for over 20 years.  Her temperature is 40 C (104 F), blood pressure is 104/63 mm Hg, pulse is 85/min, and respirations are 24/min.  Lung examination reveals lower lobe crackles with no wheezing.  Her abdomen is soft, non-distended, and non-tender.  Chest x-ray shows bilateral lower lobe interstitial infiltrates.  Which of the following additional findings is most likely to be present in this patient?

A 5-year-old boy is brought to the emergency department by his parents for severe respiratory distress.  He has not received any immunizations due to parental preference, and his medical history is otherwise unremarkable.  The patient appears very uncomfortable and anxious.  He is leaning forward and making a loud, harsh sound with every inspiration.  Temperature is 39.6 C (103.4 F).  The patient is intubated immediately.  During intubation, the epiglottis is visualized and appears markedly swollen and erythematous.  Which of the following is a major virulence factor of the pathogen causing this patient’s infection?

A 65-year-old man with chronic bronchitis comes to the emergency department due to 2 days of fever, chills, chest pain, and productive cough.  He has been hospitalized on several occasions for exacerbations of chronic bronchitis.  The patient has been advised to stop smoking but continues to smoke cigarettes.  He drinks 6–10 beers daily.  Temperature is 40.2 C (104.4 F), blood pressure is 100/70 mm Hg, pulse is 104/min, and respirations are 28/min.  On examination, he is in mild respiratory distress.  There are crackles and decreased breath sounds in the right upper lobe.  His chest x-ray shows right upper lobe consolidation.  Sputum microscopy reveals encapsulated gram-negative bacilli.  The bacteria grow pink, mucoid colonies on MacConkey agar.  Which of the following organisms is the most likely cause of this patient’s condition?

A 15-year-old boy is evaluated for fever, headache, malaise, and cough 3 weeks after returning from summer camp.  Lung auscultation is unremarkable.  Chest x-ray reveals nodular infiltrates.  The patient’s blood samples are collected for analysis in anticoagulated tubes, which are transported submerged in ice; when taken out several minutes later, turbidity and clumping are noted.  Warming the tube to body temperature leads to rapid dissolution of the clumps.  Which of the following organisms is most likely responsible for this patient’s condition?

A 21-year-old man is brought to the emergency department by his roommate due to worsening fever, headache, and confusion.  The patient recently moved to a dormitory, and his prior medical problems and immunizations are unknown.  Temperature is 39.2 C (102.6 F), blood pressure is 98/54 mm Hg, pulse is 112/min, and respirations are 24/min.  The patient appears acutely ill.  He is obtunded and has nuchal rigidity.  Leukocyte count is 19,000/mm³ with 20% band neutrophils.  Blood cultures are obtained and lumbar puncture is performed.  Gram stain of cerebrospinal fluid reveals gram-negative cocci in pairs.  Which of the following is the most likely way that this organism reached the meninges?

A 65-year-old man is admitted to the intensive care unit with second- and third-degree burns over 25% of his total body surface area.  Intravenous fluids and analgesics are administered, and his wounds are cleaned and debrided.  After 7 days, the patient develops a fever and his leukocyte count rises to 16,000 cells/µL.  Examination of the burns on his left leg shows surrounding warmth, erythema, and induration.  Gram stain of his wound tissue is shown in the image below.

Culture on MacConkey agar grows oxidase-positive colonies that do not ferment lactose.  Which of the following is the best initial treatment for this patient ?

A 38-year-old woman comes to the emergency department due to 2 days of headache, nausea, and photophobia.  She has had migraines since her early 20s but has had more frequent episodes over the last few years.  The current headache is more severe than usual, and ibuprofen did not relieve the pain.  A flulike illness preceded the symptoms.  Temperature is 38.3 C (100.9 F), blood pressure is 144/86 mm Hg, and pulse is 110/min.  The patient appears lethargic.  Cranial nerves, sensation, and motor functions are normal.  She reports neck pain when instructed to touch the chest with her chin.  No skin rash is present.  Noncontrast CT scan of the head shows no abnormalities.  Which of the following is the best next step in management of this patient?a

A pharmaceutical researcher is developing a novel agent to prevent meningococcal disease in people exposed to the bacteria.  Pili antigens from Neisseria meningitidis are extracted and used to generate antibodies that impair normal pili function.  The antibodies are administered to experimental animals with no prior exposure to N meningitidis.  These animals, along with control animals that had not received the antibodies, are subsequently exposed to the bacteria.  Compared to the control animals, a significantly greater proportion of the animals that received the antibodies remain healthy and develop no symptoms.  The efficacy of these antibodies in preventing meningococcal disease is most likely due to direct interference with which of the following microbial processes?

A 6-year-old, previously healthy girl is brought to the office due to diarrhea and abdominal pain.  The patient started having watery diarrhea after a barbeque she attended 4 days ago; her stools became grossly bloody 2 days later.  Her mother also had similar symptoms.  Temperature is 37 C (98.6 F).  The abdomen is soft with mild, generalized tenderness.  Stool culture performed on sorbitol-MacConkey agar shows sorbitol-negative colonies with biochemical properties of Escherichia coli.  Enzyme immunoassay detects an exotoxin capable of inhibiting protein synthesis in human cells.  Which of the following bacteria produces a toxin that is most similar to the one detected in this patient?

A 25-year-old woman comes to the emergency department due to 2 days of abdominal pain and bloody diarrhea.  A few days ago, she attended a church barbeque.  She does not know whether anyone else at the party developed similar symptoms.  The patient lives alone and has no medical conditions.  Temperature is 37.1 C (98.8 F), blood pressure is 119/76 mm Hg, pulse is 92/min, and respirations are 16/min.  There is abdominal tenderness with no rebound or guarding.  Guaiac-positive bloody stools are detected on rectal examination.  Stool cultures reveal an Escherichia coli strain that does not produce glucuronidase and does not ferment sorbitol on sorbitol-containing MacConkey agar, consistent with serotype O157:H7.  Which of the following best describes the mechanism of action of the toxin specific to these bacteria?

A 3-week-old boy is brought to the emergency department with fever, irritability, and poor feeding.  On examination, he is crying in his mother’s arms.  Temperature is 38.3 C (100.9 F).  The remainder of vital signs are appropriate for age.  Lung and heart sounds are clear to auscultation.  The abdomen is soft.  Capillary refill is <2 seconds in all extremities.  Cerebrospinal fluid analysis shows a leukocyte count of 600/mm³ (80% neutrophils), protein of 160 mg/dL, and glucose of 20 mg/dL; culture of the fluid grows gram-negative rods that form pink colonies on MacConkey agar.  Which of the following is the most important bacterial virulence factor during the pathogenesis this patient’s current condition?

A 24-year-old man comes to the office due to 2 days of burning pain with urination.  The patient has also had increased urinary frequency over the past few days.  He has had no fever, chills, nausea, vomiting, flank pain, or penile discharge.  The patient is sexually active with his longtime boyfriend.  Vital signs are within normal limits.  Physical examination shows mild suprapubic tenderness.  There is no costovertebral angle tenderness.  The penis is uncircumcised.  Laboratory results are as follows:

Based on the urinalysis results, which of the following organisms is the most likely cause of this patient’s illness?

A 39-year-old paraplegic man with an indwelling bladder catheter comes to the emergency department complaining of 24 hours of rigors, nausea, and vomiting.  His temperature is 38.9 C (102 F).  Physical examination shows suprapubic and costovertebral angle tenderness.  Urinalysis shows 3+ leukocyte esterase and numerous white blood cells.  Urine and blood cultures grow non-lactose-fermenting Gram-negative rods.  Which of the following pathogens is the most likely culprit?

Volunteer studies are used to determine the infectious dose of Salmonella required to cause gastroenteritis.  The curve obtained during the studies is shown in the diagram as Line 2.  Which of the following organisms is most likely represented by Line 1 in this diagram?

A 73-year-old woman comes to the physician complaining of progressive, severe pain and discharge from her left ear for the past 2 days.  She has had type 2 diabetes for many years and has been noncompliant with her medications and follow-up appointments.  On examination, moving or touching the pinna produces extreme pain.  Otoscopic examination shows granulation tissue in the left ear canal with a scant amount of discharge.  The tympanic membrane is clear, and there is no middle ear effusion.  Initial cultures from the ear show a Gram-negative rod.  Which of the following microbiological characteristics best describes the infecting organism?

A 24-year-old woman comes to the office due to spotting after vaginal intercourse.  She also has some yellow vaginal discharge and dysuria but no pelvic pain or cramping.  The patient has taken combination oral contraceptives for the past 3 years and has had no menses for the past year.  Her mother was diagnosed with cervical cancer at age 47.  BMI is 35 kg/m².  Vital signs are normal.  On examination, the abdomen is soft and nontender.  Speculum examination reveals purulent discharge from the cervical os, and the cervix is friable.  On bimanual examination, there is no cervical motion tenderness, and the adnexa are nontender bilaterally.  Urine pregnancy testing is negative.  Microscopy of the discharge shows abundant neutrophils.  If left untreated, this patient’s condition could lead to which of the following complications?

A 6-year-old, previously healthy girl is brought to the emergency department due to fever, abdominal pain, and diarrhea for the past day.  The symptoms began abruptly with high fever, as well as cramping periumbilical pain, and were quickly followed by low-volume diarrhea mixed with blood and mucus.  The patient has had no sick contacts and has not traveled recently; the family had dinner at a buffet restaurant 3 days ago.  Temperature is 38.9 C (102 F).  Physical examination reveals mild, diffuse abdominal tenderness with no guarding or rebound tenderness.  Bowel sounds are hyperactive.  Stool occult blood testing is positive, with numerous leukocytes in the stool.  Culture of stool yields gram-negative, oxidase-positive, curved rods.  Which of the following is the most likely pathogen responsible for this patient’s symptoms?

A 22-year-old male presents to the emergency department complaining of fever, abdominal pain, and vomiting.  He has had these symptoms for the past four days, but has not sought medical attention because of concerns over the cost of treatment.  On examination, the patient appears acutely ill.  There is right lower quadrant tenderness with rebound as well as a palpable mass.  CT scan of the abdomen and pelvis demonstrates a periappendiceal fluid collection.  Culture of this fluid would most likely isolate which of the following organisms?

An 8-year-old boy is brought to the emergency department due to 2 days of fever, abdominal pain, and diarrhea.  The patient has no significant medical history and has received all recommended vaccinations.  He attends a primary school and has not traveled recently.  The family recently brought home a new puppy from a kennel.  The patient’s stool is positive for occult blood and numerous leukocytes but negative for ova and parasites.  Which of the following is the most likely cause of this patient’s symptoms?

A 22-year-old woman is brought to the emergency department due to a day of fever, headaches, nausea, vomiting, and myalgias that began suddenly last night.  This morning, the patient was lethargic, difficult to arouse, and confused.  She is an exchange student who came to the United States 3 months ago, and her medical history is unknown.  Temperature is 39.4 C (102.9 F), blood pressure is 100/60 mm Hg, pulse is 112/min, and respirations are 18/min.  Physical examination shows nuchal rigidity.  Skin examination findings are shown in the image.

Leukocyte count is elevated, and lumbar puncture with cerebrospinal fluid Gram stain establishes the diagnosis.  Which of the following is the primary component of the vaccine that could have prevented this patient’s current condition?

A 32-year-old woman comes to the office due to a small amount of malodorous vaginal discharge.  She is sexually active with a male partner and uses condoms sporadically.  The patient also requests testing for all sexually transmitted diseases because she is unsure if her partner is monogamous.  She drinks a glass of wine every night with dinner and has had an abnormal Pap test in the past.  Speculum examination reveals thin, gray discharge.  Wet mount microscopy of the discharge shows large, atypical vaginal epithelial cells and no protozoa.  Application of potassium hydroxide solution to the discharge yields a strong odor.  Which of the following is the best treatment option for this patient?

A 36-year-old man comes to the emergency department after briefly losing consciousness while watching television half an hour ago.  The patient had no preceding chest pain or shortness of breath, but he has been having recurrent palpitations over the past several days.  One month ago, he noticed a non-itchy rash on his thigh that he treated with an over-the-counter steroid cream.  Medical history is otherwise unremarkable.  There is no family history of heart disease or sudden cardiac death.  The patient recently began working as a forest ranger in Pennsylvania.  He does not use tobacco, alcohol, or illicit drugs.  Blood pressure is 130/80 mm Hg and pulse is 46/min.  The lungs are clear on auscultation and heart sounds are normal.  The remainder of the examination shows no abnormalities.  ECG reveals complete atrioventricular block.  Echocardiography shows normal ventricular function with no major valvular disease.  Which of the following is the most likely underlying cause of this patient’s current condition?

A 23-year-old woman comes to the emergency department for evaluation of right lower abdominal pain and bloody vaginal discharge.  Her symptoms began this morning and are progressively worsening.  She is sexually active with 1 male partner.  The patient takes no daily medications and has no known drug allergies.  Her last menstrual period was 5 weeks ago, and she has never been pregnant.  Blood pressure is 112/70 mm Hg while supine and 96/60 mm Hg while standing.  The patient is tachycardic and ill appearing.  On pelvic examination, the uterus is small and mobile, and there is right adnexal tenderness.  Laboratory results are as follows:

This patient’s clinical presentation is most likely due to a prior infection with which of the following pathogens?

A 55-year-old man comes to the emergency department with fever and chills.  He has a history of acute myeloid leukemia for which he is receiving chemotherapy.  His temperature is 38.3 C (101 F), blood pressure is 90/50 mm Hg, pulse is 124/min, and respirations are 22/min.  Physical examination is notable for multiple skin patches with necrotic centers and occasional ulcerations, as shown in the image below.

Laboratory results are as follows:

Blood cultures are drawn.  Which of the following organisms is most likely responsible for this patient’s symptoms?

A 26-year-old man comes to the office due to 2 days of rash on his right thigh.  It is mildly pruritic and not painful.  He has no chronic medical conditions but occasionally takes ibuprofen for shoulder pain following intensive workouts at the gym.  Temperature is 37.2 C (98.6 F), blood pressure is 115/78 mm Hg, and pulse is 94/min.  The rash is shown in the image below:

No other lesions are seen.  Which of the following is the most likely underlying cause of this patient’s rash?