A 35-year-old man comes to the office due to 3 months of progressive swelling under his left jaw.  The swelling spontaneously opened and drained pus a month ago but has not improved or resolved.  The patient has no medical conditions other than dental caries; he had a tooth extracted prior to symptom onset.  Physical examination shows an indurated, nontender mass in the left submandibular area with a small opening on the overlying skin.  Gentle pressure on the mass yields thick pus containing sand/grain-like particles.  The microbial pathogen with which of the following characteristics is the most likely cause of this patient’s condition?

A 74-year-old man is hospitalized with acute urinary retention complicated by urosepsis.  He has a urinary catheter placed and is started on intravenous antibiotics.  The patient requires endotracheal intubation and mechanical ventilation for 24 hours but is extubated successfully on the second hospitalization day.  On the third day of hospitalization, he develops fever and right jaw pain.  Physical examination shows firm swelling of the preauricular area on the right side extending to the angle of the mandible.  Which of the following serum markers is most helpful for confirming the diagnosis?

A 35-year-old man is admitted to the hospital after he fell off a moving tractor and caught his right leg underneath the vehicle.  The patient has no past medical history.  Over the next few hours, he develops worsening severe pain and rapidly increasing swelling in the injured right leg.  Crepitus is present on physical examination.  Emergent radiographic imaging reveals gas in the tissues.  Surgical evaluation is performed.  The microorganism most likely responsible for this patient’s condition is also commonly associated with which of the following conditions?

A 4-year-old boy is brought to the office due to skin lesions on his chin for the past 2 days.  Temperature is 36.1 C (97 F).  The patient is well-appearing.  There are mildly tender perioral papules and pustules, some of which appear to have broken down and are covered with amber-colored crusting.  A wound culture is taken, and Gram stain shows gram-positive cocci in chains.  Which of the following symptoms would be most likely to develop as a potential complication of this patient’s infection?

A group of researchers collected demographic and clinical data from multiple hospitals across the nation to determine epidemiological features of nosocomial bloodstream infections.  In this study, a nosocomial bloodstream infection was diagnosed if blood cultures drawn 48 hours after admission yielded a pathogenic organism.  Analysis of the data collected over several decades shows that staphylococci are increasingly responsible for the identified cases.  An increased use of which of the following is most likely the underlying cause of the observed trend?

A 65-year-old man comes to the emergency department with a sore on his right hand.  The lesion began as a small, painless papule 3 days ago and rapidly enlarged and ulcerated.  The patient has had no injury or insect bite, has no prior medical conditions, and takes no medications.  He is in the wool business and recently returned from a tour of wool-processing plants in several Asian countries.  Physical examination shows a necrotic ulcer with a black eschar on the dorsum of the right hand.  There is extensive edema of surrounding tissues.  The toxin causing edema around this patient’s ulcer has a mechanism of action most similar to a different toxin produced by which of the following bacteria?

A 34-year-old man comes to the emergency department due to 2 days of fever, malaise, and myalgia.  He has also had worsening shortness of breath and cough.  The patient works in the mail room at a large company, does not smoke cigarettes, and drinks alcohol occasionally.  Chest imaging shows lung infiltrates and prominent widening of the mediastinum.  Sputum and blood cultures yield large, gram-positive rods that form colonies resembling a “Medusa head.”  Which of the following is the most likely bacterial virulence factor contributing to this patient’s condition?

A 24-year-old female presents to your office with burning urination, urgency and frequency.  She is sexually active.  Urine cultures show catalase-positive, gram-positive cocci.  The organism responsible for this patient’s symptoms is most likely to be:

A 22-year-old man comes to the office due to a painful lesion on his upper back.  The patient developed a small tender papule a few days ago that progressively enlarged and became red and painful.  He has had 3 similar lesions on his upper back over the past 6 months, which were treated with incision and drainage and antibiotics.  The patient has no other medical issues and takes no medications.  He does not use tobacco, alcohol, or illicit drugs.  On physical examination, there is a 3-cm tender pustule with surrounding erythema on his right upper back.  Gram staining of purulent drainage from the lesion is shown below.

Which of the following most likely predisposed this patient to recurrent infections?

A 54-year-old man comes to the emergency department due to fever, chills, and malaise over the past day.  His other medical problems include recently diagnosed Hodgkin disease.  Temperature is 39.4 C (103 F), blood pressure is 102/61 mm Hg, and pulse is 114/min.  The patient appears ill.  Blood cultures grow motile gram-positive rods that produce a very narrow zone of beta hemolysis on blood agar.  The bacteria are also found to grow well at refrigerated temperature.  Which of the following processes is most important for eliminating these bacteria from the body?

A 6-day-old boy is brought to the emergency department due to inability to feed for 12 hours.  The parents report that he has been unable to open his mouth and has had clenched hands all day.  The neonate was delivered at home by the grandmother.  After the umbilical cord was cut with a kitchen knife, the parents rubbed a coin on the umbilical stump to promote wealth during the child’s life as is customary in their culture.  The mother did not undergo prenatal care but says the pregnancy was uncomplicated.  Physical examination shows increased muscle tone throughout, arching of the back, and dorsiflexed feet.  In addition to hygienic umbilical cord care, which of the following interventions would have most likely prevented this neonate’s condition?

A microbiologist is studying the structure and function of bacterial cell walls.  In an experiment, group A streptococci are treated with chemicals to solubilize the cell wall.  Various cell wall–associated proteins are subsequently extracted.  Electron microscopic evaluation of a specific protein shows structural homology with mammalian tropomyosin and myosin.  This protein acts as a virulence factor for certain species of the organism.  Which of the following is the most likely function of this bacterial cell wall–associated protein?

A 12-year-old boy is brought to the office with fever, chills, and a rash that appeared this morning.  Two days earlier, the child started complaining of a sore throat.  His temperature is 38.3 C (101 F).  Examination shows a diffuse erythematous rash on his chest and abdomen that blanches with pressure, along with numerous 1- to 2-mm papules.  The throat is erythematous with gray-white tonsillar exudates and the tongue is bright red.  Which of the following complications is most likely to develop in this patient?

A 45-year-old man comes to the physician due to pain, swelling, and erythema affecting his right leg.  He says that he suffered a minor cut to his leg a few days ago while cleaning his garage.  Physical examination shows an indurated region surrounding a minor laceration that is draining a purulent exudate.  He is diagnosed with cellulitis and started on the appropriate treatment.  Gram stain of the exudate shows gram-positive cocci in clusters.  The organism most likely responsible for this patient’s infection synthesizes a protein as part of its peptidoglycan cell wall that does which of the following?

A 24-year-old man develops mild, nonexudative pharyngitis shortly after travel to Eastern Europe.  Throat swab culture in tellurite-containing, selective media yields black, iridescent colonies.  Microscopy shows club-shaped, gram-positive bacilli in a palisade arrangement, which are identified as nonpathogenic Corynebacterium diphtheriae.  These bacteria can acquire virulence to become pathogenic through which of the following mechanisms?

An 8-year-old boy is brought to the emergency department due to 24 hours of fever and chills.  Temperature is 38.3 C (101 F).  On examination, the patient is lethargic with dry mucous membranes.  There are no oropharyngeal lesions, and tympanic membranes are normal.  The lungs are clear on auscultation, and heart sounds are normal.  The abdomen is soft and nontender.  There is no skin rash.  Laboratory testing shows a leukocyte count of 24,000/mm³ with 15% bands, 80% neutrophils, and 5% lymphocytes.  Peripheral blood smear is shown in the exhibit.  Blood cultures are obtained and intravenous fluids and empiric antibiotics are initiated.  The patient’s blood cultures are most likely to grow which of the following organisms?

A 58-year-old woman comes to the office for a health checkup prior to starting volunteer work at a hospital.  She has a history of hypothyroidism and takes levothyroxine.  The patient feels well, and review of systems is negative.  She does not use tobacco, alcohol, or illicit drugs.  Her examination findings are unremarkable.  During a laboratory test, her white blood cells are incubated with mycobacterial antigens.  Compared to the control, a large amount of interferon-gamma is detected in her blood sample.  Which of the following cell types is most directly responsible for this finding?

A 65-year-old woman with ovarian cancer undergoing chemotherapy is hospitalized due to fever, headache, ataxia, and progressive confusion.  Two days later, a 26-year-old pregnant woman is admitted to the same hospital with fever, nausea, vomiting, diarrhea, and muscle aches and spontaneously delivers a preterm infant who has generalized skin rash.  Laboratory evaluation reveals that these patients’ conditions were caused by the same microbial organism.  The state health department is notified.  Reports of similar illnesses are noted in neighboring cities.  A subsequent investigation reveals that 8 people were affected, all of whom consumed food products from a single processing facility.  Which of the following organisms is most likely responsible for this outbreak?

A 6-hour-old boy is evaluated in the newborn nursery due to temperature instability, grunting, and lethargy.  The patient was born at full term to a 21-year-old woman who came to the emergency department in active labor with ruptured membranes; she had not received adequate prenatal care.  On physical examination, the boy is hypothermic, tachycardic, and tachypneic, and he has mottled skin.  Blood samples are obtained, and empiric antibiotics are initiated for the treatment of sepsis.  Cultures on blood agar grow bacterial colonies with a narrow zone of clear hemolysis.  When these bacteria are grown in the presence of Staphylococcus aureus, the zone of hemolysis is enhanced around the staphylococcal colonies.  Which of the following is the most likely pathogen infecting this patient?

A 62-year-old man comes to the emergency department due to 3 days of severe sore throat, difficulty swallowing, and hoarseness.  He has no chronic medical conditions but has not seen a physician for many years.  The patient received recommended childhood vaccinations but has had none since then.  He is a farmer and has not traveled recently but has been in contact with migrant farmworkers.  Temperature is 38.3 C (101 F), blood pressure is 110/60 mm Hg, and pulse is 116/min.  Physical examination reveals pharyngeal erythema and edema with coalescing, gray exudate.  Bilateral cervical lymph nodes are enlarged and tender.  A rapid streptococcal test result is negative.  Throat swab culture grows non-spore–forming, gram-positive rods.  Which of the following is most helpful in preventing the systemic complications of this infection in this patient?

A 34-year-old man is admitted to the intensive care unit due to fever, chills, shortness of breath, and altered mental status.  His symptoms began 3 days ago and have progressively worsened over the last 24 hours.  His past medical history is significant for a motor vehicle accident 2 years ago in which he sustained blunt abdominal trauma and required emergency laparotomy due to internal bleeding.  His blood pressure is 81/44 mm Hg and pulse is 122/min.  He is started on broad-spectrum antibiotics, intravenous fluids, and vasopressors.  His condition continues to deteriorate and he dies in the hospital several hours later despite extensive resuscitation efforts.  Blood cultures obtained on admission grow Streptococcus pneumoniae.  Impairment of which of the following mechanisms most likely contributed to the severity of this patient’s infection?

A 67-year-old man comes to the emergency department with low-grade fevers and fatigue.  He has no history of heart disease.  His temperature is 38.4 C (101.1 F), blood pressure is 117/76 mm Hg, pulse is 82/min, and respirations are 14/min.  On examination, there is a diastolic murmur at the left sternal border.  He has nontender erythematous macules on his hands and feet.  Blood cultures show gram-positive cocci that are catalase-negative and able to grow in hypertonic (6.5%) saline and bile.  This patient’s medical history is most likely to include which of the following procedures within the past month?

A 52-year-old man is hospitalized due to 2 weeks of low-grade fever, malaise, anorexia, and fatigue.  The patient has a history of bicuspid aortic valve and underwent aortic valve replacement a year ago.  Physical examination reveals a new regurgitation murmur.  Blood cultures repeatedly grow gram-positive cocci in clusters, which are identified as Staphylococcus epidermidis.  This pathogen most likely demonstrates which of the following characteristics?

An 8-year-old boy with acute lymphoblastic leukemia receiving maintenance chemotherapy is hospitalized due to fever and neutropenia.  Treatment with broad spectrum intravenous antibiotics is begun and the patient’s fever resolves.  His neutrophil count increases with granulocyte colony-stimulating factor therapy and blood cultures remain negative.  On day 5 of hospitalization, the patient experiences nausea, abdominal cramping, and 6-8 episodes of watery diarrhea.  Temperature is 37.7 C (99.9 F).  Physical examination shows mild lower abdominal tenderness without guarding or rebound tenderness.  Stool testing for occult blood is negative.  Which of the following is the single best stool test to establish the cause of this patient’s diarrhea?

A 27-year-old man comes to the emergency department due to several hours of right foot pain and swelling.  The patient says he was working in his barn last night and stepped on an old nail.  This morning, he awoke with pain near the injury site.  It has increased throughout the day and is accompanied by progressive swelling.  The patient has no chronic medical conditions and takes no medications.  Temperature is 38.1 C (100.6 F), blood pressure is 135/75 mm Hg, and pulse is 95/min.  The right foot is swollen with some erythema around the injury site.  Radiographic imaging reveals gas in the tissues.  Surgical exploration shows extensive tissue necrosis.  Culture from the site reveals gram-positive rods.  Which of the following best describes the mechanism of action of the toxin responsible for the necrotic effects seen in this patient?

A 79-year-old woman is brought to the emergency department from the nursing home due to explosive diarrhea.  Laboratory studies show leukocytosis, and results from a stool specimen return positive for Clostridioides difficile.  Three days later, one of the nurses who cared for the patient at the nursing home is admitted with diarrhea and is found to have C difficile infection.  However, the other nurses who also took care of the patient are asymptomatic and do not develop the infection.  If all the nurses were similarly exposed to C difficile, which of the following is the most likely reason that the asymptomatic nurses did not develop the infection?

A 10-year-old boy is brought to the emergency department after experiencing high fevers and chills for the last few days.  He also complains of dull pain just above his left knee.  He has no history of recent trauma other than minor scrapes to his knees and elbows while playing outside.  Physical examination shows point tenderness 3 cm above the kneecap.  There is no joint effusion.  Radiographs show soft-tissue swelling and a periosteal reaction over the lower end of the femur.  Which of the following organisms is most likely responsible for this patient’s symptoms?

A 57-year-old woman is hospitalized with high-grade fevers, chills, and right upper abdominal pain.  The patient lives in Wisconsin and has no history of international travel.  Her temperature is 40 C (104 F).  Physical examination of the abdomen shows rebound tenderness.  Laboratory studies show elevated aspartate and alanine aminotransferases.  Imaging studies reveal a fluid-filled cavity within the right lobe of the liver as shown below.

Which of the following microorganism/route combinations is most likely to be the cause of this patient’s infection?

A 5-month-old boy is brought to the emergency department due to poor feeding, weakness, and complete loss of extremity muscle tone.  Vaccinations are up to date, and there is no significant medical history.  The patient receives formula as his sole source of nutrition except for occasional fruit juice and honey.  Which of the following is most likely to confirm the diagnosis in this patient?

A 52-year-old active smoker comes to the office due to a 2-week history of malaise, shortness of breath, and decreased appetite.  The patient has a history of bicuspid aortic valve and hypertension.  His temperature is 39 C (102.2 F), blood pressure is 140/80 mm Hg, and pulse is 110/min.  On examination, the patient has a systolic ejection murmur at the right upper sternal border that is unchanged from previous visits; there is a new decrescendo diastolic murmur best heard in the third intercostal space at the left sternal border.  Laboratory studies show hemoglobin is 9.3 g/dL and leukocytes are 16,500/mm³.  Blood cultures are positive for Streptococcus gallolyticus (formerly S bovis).  Additional workup for this patient should focus on which of the following conditions?

A 1-year-old boy is brought to the emergency department by his mother because of high fever, irritability, and photophobia.  He recently underwent ventriculoperitoneal shunt placement for congenital hydrocephalus.  On physical examination, his temperature is 38.3° C (101° F), and passive flexion of the neck results in spontaneous flexion of the hips and knees.  Blood cultures grow coagulase-negative Staphylococcus.  Which of the following is the most important virulence mechanism by which this bacterium causes disease?