A 22-year-old woman comes to the emergency department due to intermittent fevers and chills over the past few days.  Last week, she underwent routine dental cleaning with no complications.  The patient has a history of mitral valve prolapse.  Temperature is 38.2 C (100.8 F).  Physical examination reveals a non-ejection mid-systolic click followed by a late-systolic murmur best heard at the cardiac apex.  Blood cultures are drawn and grow gram-positive bacteria that synthesize dextrans from sucrose.  Which of the following would be the most likely adherence site for these bacteria?

A 54-year-old woman is brought to the emergency department after a generalized tonic-clonic seizure.  The patient had fever, night sweats, fatigue, and productive cough during the previous week.  She underwent a renal transplant 2 years ago and takes immunosuppressive therapy.  On examination, she is febrile and postictal.  There are scattered lung crackles and normal heart sounds.  Leukocyte count is elevated, and chest x-ray reveals patchy bronchopneumonia with several nodules.  Brain imaging shows multifocal abscesses.  Sputum Gram stain reveals filamentous, branching gram-positive rods as shown below:

This pathogen is likely to demonstrate which of the following characteristics?

A 52-year-old man comes to the office with 3 weeks of cough, night sweats, and occasional hemoptysis.  The patient also has type 2 diabetes mellitus and hypertension.  He emigrated from Vietnam 15 years ago to start a job as an associate professor at a university.  He has a 20 pack-year smoking history but quit 5 years ago.  Sputum cultures grown on a selective medium demonstrate mycobacteria that grow in parallel chains forming serpentine cords under microscopy.  This observed growth pattern most strongly correlates with which of the following bacterial characteristics?

Bacteria isolated from the lung tissue of a 32-year-old patient fail to decolorize with hydrochloric acid and alcohol after staining carbolfuchsin.  Which of the following cell wall components is most likely responsible for this staining phenomenon?

A 2-day-old boy is evaluated in the neonatal intensive care unit for lethargy and respiratory distress.  The patient was born at home via spontaneous vaginal delivery at term gestation to a mother who did not receive routine prenatal care.  Temperature is 38.9 C (102 F) and pulse is 162/min.  Physical examination shows tachypnea and grunting.  Blood cultures grow beta-hemolytic, gram-positive cocci in chains that elicit a narrow zone of complete hemolysis on blood agar.  Which of the following virulence factors most likely contributed to the development of this patient’s current condition?

A 34-year-old man with HIV comes to the emergency department due to fever, chills, productive cough, and left-sided chest pain that worsens with deep breathing.  His symptoms began 3 days ago and have progressively worsened.  The patient was diagnosed with HIV 4 years ago during routine screening, and he takes antiretroviral therapy.  His CD4 count was 480 cells/mm³ a month ago.  Temperature is 38.8 C (102 F), blood pressure is 110/66 mm Hg, pulse is 110/min, and respirations are 22/min.  Physical examination shows dullness to percussion, bronchial breath sounds, and crackles over the left lower lung.  The right lung and cardiac auscultation findings are normal.  Laboratory testing reveals an elevated leukocyte count with left shift.  Which of the following organisms is most likely responsible for this patient’s current symptoms?

A 78-year-old man is brought to the emergency department due to fever, cough, and shortness of breath.  The patient recently moved into an assisted living facility after living with his family that owned several pets.  He has a 40-pack-year smoking history.  Temperature is 39.4 C (103 F), blood pressure is 106/62 mm Hg, pulse is 112/min, and respirations are 28/min.  There is dullness to percussion and bronchial breath sounds over the left lung.  Chest x-ray reveals a left lower lobe consolidation.  Sputum microscopy shows gram-positive diplococci.  Which of the following would have been most helpful in preventing this patient’s lung infection?

A 63-year-old man comes to the emergency department due to fever and cough.  A medical student interviews and examines the patient and notes that he has had 3 months of fevers, night sweats, and productive cough.  Chest x-ray reveals a right apical lung lesion, and the patient is subsequently placed in respiratory isolation.  The medical student was in close contact with the patient during the initial interview and the patient coughed frequently during their interaction.  The student had a negative tuberculin skin test prior to beginning the rotation.  If this patient’s infectious disease is transmitted to the student, which of the following processes is most likely to occur during the first week after exposure?

A 40-year-old woman is evaluated for neck pain that has gradually worsened over the past several months.  The patient reports a pulling sensation in her neck that causes difficulty in keeping her head straight.  She has had no trauma and has no other medical conditions.  Physical examination shows a spastic and thickened sternocleidomastoid muscle causing head tilt.  The patient is diagnosed with idiopathic torticollis.  She is treated with injections of a bacterial product into the affected muscle, with significant relief of her symptoms.  The substance administered to this patient is derived from bacteria that most likely demonstrate which of the following characteristics?

A 55-year-old man comes to the office due to malaise and cough over the past 2 months.  He describes yellow sputum production with occasional streaks of blood.  The patient smokes a pack of cigarettes daily and has a history of alcohol use disorder with prior episodes of binge drinking.  Temperature is 37.1 C (98.8 F).  Examination shows poor dentition with dental caries, gingivitis, and enlarged submandibular lymph nodes.  Coarse rhonchi are heard during auscultation of the right lung.  Chest CT scan reveals an extensive right lung consolidative process with air bronchograms.  Bronchoscopy is performed, and a lung biopsy specimen shows the findings in the image below.

Which of the following is the most likely diagnosis?

A 58-year-old man comes to the emergency department with a 1-day history of fever, headache, and vomiting.  He underwent kidney transplantation last year.  His temperature is 39.4 C (103 F), blood pressure is 130/80 mm Hg, pulse is 98/min, and respirations are 16/min.  Neck stiffness is present on physical examination.  Cerebrospinal fluid (CSF) results are as follows:

Gram-positive rods with tumbling motility at room temperature are seen on CSF microscopy.  How was this patient’s infection most likely transmitted?

A 42-year-old homeless man comes to the emergency department due to painful muscle spasms.  The patient has had frequent involuntary contractions of the jaw, neck, and trunk muscles, which are triggered by sensory stimuli.  He has a history of injection drug use and has had several injection site infections.  Physical examination shows neck stiffness and decreased opening of the jaw.  Which of the following is the most likely path taken by the agent responsible for this patient’s neurologic symptoms?

A 64-year-old man comes to the office due to 2 days of dysuria, urinary frequency, and urgency.  He recently underwent a cystoscopy for evaluation of hematuria.  The patient has a history of hypertension, type 2 diabetes mellitus, and 30 pack-years of cigarette smoking.  His temperature is 38.2 C (100.8 F).  On examination, there is suprapubic tenderness on deep palpation but no costovertebral angle tenderness.  Urinalysis findings are as follows:

Midstream urine culture grows gram-positive cocci in chains with growth >100,000 colony-forming units/mL.  The organism responsible for this patient’s condition is most likely to demonstrate which of the following?

A 44-year-old man is brought to the emergency department by his wife because of confusion that began a few hours ago.  Earlier in the day, he complained of a severe headache and nausea.  On physical examination, his temperature is 38.3 C (101 F) and nuchal rigidity is present.  Lumbar puncture and cerebrospinal fluid (CSF) examination show:

Gram stain of the CSF would most likely reveal which of the following?

A 34-year-old woman comes to the office due to skin changes.  Blotches of skin on her arms appear to be of a different color, and she also has a tingling sensation in her hands.  These symptoms have developed over the past several months.  The patient has no other past medical history.  She is a political refugee from East Africa.  On examination, there are patchy areas of skin anesthesia and hypopigmentation on her upper extremities.  Nerve biopsy evaluated under light microscopy shows many organisms invading Schwann cells.  HIV testing is negative.  Which of the following organisms is the most likely cause of this patient’s condition?

A 24-year-old man comes to the emergency department due to severe nausea and vomiting 2 hours after a church picnic.  He ate chicken salad that had been sitting out in the sun.  The patient has no prior medical conditions.  Temperature is 37.2 C (98.9 F), blood pressure is 126/84 mm Hg, and pulse is 86/min.  No abdominal tenderness is present, and bowel sounds are normal.  Blood cell counts and serum chemistry studies are within normal limits.  Symptoms improve significantly within several hours with supportive care.  Which of the following is the most likely cause of this patient’s symptoms?

A 6-month-old boy is brought to the emergency department due to poor feeding, irritability, and rash.  He was well-appearing until 2 days ago, when he developed little interest in drinking and a progressive rash.  Physical examination reveals diffuse erythema and desquamation that is most prominent at the neck, axillae, inguinal folds, and perioral region.  The perioral area also has crusting and the lips are dry and cracked, but the mucosal membranes are normal.  The epidermis easily comes off with gentle pressure at the erythematous areas.  Which of the following is the most likely cause of this patient’s symptoms?

A 68-year-old woman with end-stage renal disease receiving intermittent hemodialysis comes to the office due to back pain.  She says that she may have “pulled a muscle” while getting out of her car a week ago; since then she has had progressively worsening back pain.  She has no leg numbness or weakness but reports malaise and fatigue.  The patient also has hypertension and type 2 diabetes mellitus and was treated several weeks ago for staphylococcal bacteremia associated with the dialysis catheter.  Her temperature is 38 C (100.4 F).  On examination, she has tenderness over the upper lumbar vertebrae without overlying skin changes.  The straight leg raise test is negative.  Which of the following is the best next step in management of this patient?

A 19-year-old woman comes to the emergency department with pelvic pain, fever, and chills that began last night.  The patient’s last menstrual period was 12 weeks ago, and she had a surgical pregnancy termination at a clinic 2 days ago.  Temperature is 38.3 C (100.9 F), blood pressure is 92/60 mm Hg, and pulse is 102/min.  Physical examination shows diffuse lower abdominal tenderness without rebound or guarding.  Speculum examination reveals an open cervical os and malodorous tissue in the vaginal canal.  Bimanual examination shows moderate uterine tenderness and no adnexal masses.  Which of the following is the most likely cause of this patient’s condition?

A 23-year-old woman comes to the hospital due to fever, chills, dyspnea, and cough for the past several days.  She also has sharp right-sided chest pain exacerbated by breathing.  Her temperature is 39 C (102 F), blood pressure is 115/70 mm Hg, and pulse is 108/min.  On examination, the patient has several needle track marks on both arms.  There is a 3/6 holosystolic murmur heard best at the lower sternal border that increases on inspiration.  Chest imaging reveals scattered bilateral peripheral lung opacities.  HIV testing is negative.  This patient’s blood cultures are most likely to grow which of the following organisms?

A 46-year-old man is brought to the emergency department due to worsening blurry vision, difficulty swallowing, and dry mouth for the past hour.  The patient has also been having nausea and abdominal discomfort but no headache or fever.  He has no prior medical conditions and takes no medications.  The patient has not traveled recently, but he attended a community potluck event yesterday where he sampled home-canned foods.  Physical examination shows bilateral ptosis and sluggish pupillary response.  Which of the following steps in neuromuscular transmission is most likely affected in this patient?

A 65-year-old man comes to the emergency department due to abdominal pain and diarrhea. Three weeks ago, he drove from Texas to Mexico for a family vacation. Temperature is 38.3 C (101 F), blood pressure is 115/70 mm Hg, and pulse is 98/min. Abdominal examination shows mild, generalized tenderness with no rebound tenderness or guarding. Leukocyte count is 14,000/mm3. Sigmoidoscopy demonstrates white-yellow plaques on the colonic mucosa, and biopsy shows that the plaques are composed of fibrin and inflammatory cells. Further questioning regarding this patient’s trip to Mexico is most likely to reveal which of the following events?