A 35-year-old man who received a kidney transplant one year ago comes to the emergency department with pleuritic chest pain and cough.  He is found to have a low-grade fever.  Chest x-ray reveals an infiltrate in the lower lobe of the right lung.  The patient is started on broad-spectrum antibiotics, including vancomycin, ceftriaxone, and azithromycin.  Despite these measures, his condition worsens over the next several days.  Bronchoscopy with bronchoalveolar lavage is performed.  Mucicarmine staining of his bronchoalveolar fluid reveals the following on light microscopy:

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

A 32-year-old woman comes to the emergency department due to 4 weeks of worsening cough, shortness of breath, and low-grade fevers.  She also has had an unintentional weight loss of 4.5 kg (10 lb) over the past 3 months.  Temperature is 37.1 C (98.8 F), blood pressure is 120/70 mm Hg, pulse is 102/min, and respirations are 22/min.  Pulse oximetry shows 86% on ambient air.  Lung auscultation reveals bilateral crackles.  There are no heart murmurs, and jugular venous pressure is normal.  Chest x-ray reveals diffuse interstitial infiltrates.  The patient undergoes bronchoscopy; methenamine silver staining of the bronchoalveolar lavage specimen is shown in the image.

Which of the following is the most appropriate pharmacotherapy for this patient’s current condition?

An 8-year-old girl is brought to the office for evaluation of a skin rash on her upper back.  The rash is not painful but is mildly pruritic.  She has no prior medical conditions, takes no medications, and is up to date with vaccinations.  Vital signs are within normal limits.  Right scapular area skin examination is shown in the exhibit.  The remainder of the examination is normal.  Which of the following is the most likely cause of this patient’s current condition?

A 32-year-old man presents with low grade fever, cough and generalized malaise.  The patient has otherwise been healthy and has no significant medical history.  His past surgical history includes tonsillectomy and tympanostomy tube placement at the age of 8.  Careful questioning reveals that the patient recently came back from an “outdoor adventure” trip in the Great Lakes, and began feeling sick only after returning.  Microscopic examination of a KOH preparation of an exudate specimen reveals a large yeast cell with a single bud.  When incubated in a specific medium at 75.2ºF (24ºC), a multicellular structure with branching, tubular cells grows.  The patient is most likely infected with which of the following:

A 46-year-old man comes to the office due to skin lesions on his left hand and arm that developed over the past several days.  He first noticed a small papule on his left hand that has increased in size.  Additional nodules then appeared proximally on his arm.  The lesions are not particularly painful.  The patient has never had similar symptoms.  He has had no fever and otherwise feels fine.  He has no significant medical history, takes no medications, and has no allergies.  Temperature is 37.1 C (98.8 F).  The lesions are shown in the image below.

Fluid aspirated from a lesion is examined microscopically, and several elongated yeast cells with narrow-based buds are visualized.  Which of the following risk factors is most likely a component of this patient’s history?

A 28-year-old man comes to the office due to a rash on the lower extremity.  The patient noticed a pruritic papule on the dorsum of his right foot just prior to returning from a south Florida beach 2 days ago.  He walked barefoot on the beach but does not recall any trauma.  Since his return home, the rash and itching have progressively worsened.  The patient has no other medical problems and takes no medications.  Physical examination findings are shown below.

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

A 38-year-old man is brought to the emergency department after a generalized tonic-clonic seizure.  He had several days of progressive headache, vomiting, and confusion prior to the episode.  Temperature is 38 C (100.4 F).  Physical examination shows a lethargic patient with bilateral papilledema and neck rigidity.  His condition progressively worsens, and the patient dies in the hospital several days later.  An autopsy is performed, and the brain histopathology is shown below.

Which of the following pathogens is most likely responsible for these findings?

A 28-year-old man comes to the emergency department due to 2 weeks of headaches, fever, and episodes of inattentiveness.  The patient has a history of Hodgkin lymphoma and is undergoing treatment with combination chemotherapy.  Temperature is 38.2 C (100.8 F), blood pressure is 138/84 mm Hg, and pulse is 72/min.  Lumbar puncture reveals an elevated opening pressure, and cerebrospinal fluid analysis shows moderately increased protein levels and low glucose levels.  Light microscopy of the cerebrospinal fluid is shown in the image.

Which of the following should be included in initial treatment for this patient’s current condition?

A 60-year-old woman comes to the physician with 10 days of diffuse itching.  A rash began on her hands and quickly spread to involve her wrists, arms, and axillae.  She has difficulty sleeping and complains of intense itching in these areas.  She works in a day care center and primarily takes care of children age 2-5.  The patient has no other medical conditions and takes no chronic medications.  Vital signs are normal.  Her BMI is 24 kg/m².  Skin examination shows small erythematous papules on the palms with excoriations, scattered vesicles, and pustules on the finger webs, palms, and wrist creases, as seen in the image below.  She has similar lesions in her axillary skin folds bilaterally.  The remainder of the examination is within normal limits.

Which of the following is the most likely finding on skin sampling of this patient’s lesions?

A 2-month-old boy is brought to the clinic for irritability, poor feeding, and frequent vomiting.  Weight is at the 15th percentile, and head circumference is at the 96th percentile.  Lung sounds are clear throughout, and heart auscultation reveals no murmurs.  The abdomen is soft and nondistended.  There is hepatosplenomegaly.  Diffusely reduced muscle tone is present in all extremities.  Funduscopy reveals white-yellow chorioretinal lesions in both eyes.  MRI of the brain shows enlarged ventricles and scattered intracranial calcifications.  Which of the following is the most likely cause of this patient’s condition?

A 54-year-old man is brought to the emergency department due to a week of progressive headaches and confusion.  He was treated for viral esophagitis 6 months ago and Pneumocystis pneumonia 2 months ago.  Temperature is 38 C (100.4 F), blood pressure is 142/86 mm Hg, and pulse is 64/min.  The patient is lethargic and oriented to person and place only.  CT scan of the head shows a mildly increased ventricle size but no other abnormalities.  Lumbar puncture reveals an elevated opening pressure, and cerebrospinal fluid analysis shows moderately increased protein concentration and pleocytosis.  The latex agglutination test is positive for soluble polysaccharide antigen.  Light microscopy of this patient’s cerebrospinal fluid is most likely to reveal which of the following?

A 36-year-old man comes to the office to discuss a skin rash on his shoulders and upper chest.  The patient first noticed the rash 2 months ago while vacationing at a beach resort.  The rash has worsened progressively and is associated with mild pruritus, but he has no other associated symptoms.  The patient is a road construction worker, and often works in hot, humid areas.  Past medical history is notable for seasonal allergies and childhood asthma.  He does not use tobacco or alcohol.  Skin examination findings are shown in the image below.

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

A 42-year-old woman is brought to the office due to 2 weeks of personality changes.  Her family reports that she has become withdrawn and does not talk as much as before.  She has also had difficulty picking things up with her right hand.  The patient has HIV infection and does not take her antiretroviral therapy consistently.  She is a former smoker and used injection drugs in the past.  The patient works in a food-processing facility and has not traveled recently.  Temperature is 37 C (98.6 F), blood pressure is 126/78 mm Hg, pulse is 82/min, and respirations are 14/min.  The patient is oriented to time, place, and person but is slow to respond to questions.  Motor strength is decreased in her right upper extremity, with an exaggerated biceps reflex.  CD4 count is 78 cells/mm³ and plasma HIV RNA is 400,000 copies/mL.  CT scan of the head shows a small hypodense area in the left frontal lobe with ring enhancement after intravenous contrast administration.  Two similar lesions are present in the basal ganglia and cerebellum.  The infectious agent directly responsible for this patient’s neurologic lesions is most likely transmitted by which of the following mechanisms?

A 38-year-old woman comes to the office for evaluation of a skin lesion.  It began as a pink papule on her right arm and has progressively enlarged over several weeks.  The lesion is not pruritic or painful and has not been associated with fever or other systemic symptoms.  The patient returned from a trip to Costa Rica 5 weeks ago.  During her travel, she stayed at a beachside resort and hiked in the rainforest.  Skin examination findings are shown below.  Biopsy of the lesion reveals intracellular, round-oval protozoa with rod-shaped kinetoplasts.  Which of the following most likely led to this patient’s current condition?

A 42-year-old man is brought to the emergency department following a generalized tonic-clonic seizure witnessed by his family.  He has no history of seizures but has had headaches for the past several weeks.  The patient works in construction.  He lives in the United States and has not traveled recently.  The patient has never had pets.  He smokes a pack of cigarettes daily and drinks 3 or 4 cans of beer every evening.  Physical examination is notable for a postictal state, oral thrush, and several enlarged cervical and inguinal lymph nodes.  MRI of the brain with contrast is shown in the image below.

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

A 34-year-old woman comes to the office due to vulvar and perianal itching and vaginal discharge.  She has no other concerns.  The patient is sexually active with her fiancé and takes combined estrogen-progestin oral contraceptive pills daily.  She has regular monthly menstrual cycles and does not use barrier contraception.  The patient took antibiotics 2 weeks ago for a urinary tract infection but has no chronic medical conditions.  Pelvic examination shows an erythematous vulva and perineum with excoriations.  A thick, white discharge is present that adheres to the vaginal walls.  The remainder of the physical examination is normal.  Which of the following is the most likely underlying cause of this patient’s current condition?

A 30-year-old man is admitted to the intensive care unit after his wife found him unconscious at home.  She says that he has lost a significant amount of weight recently despite no significant change in dietary habits.  Laboratory studies show a serum pH of 7.1 and positive serum ketones.  His mucus membranes are extremely dry.  Treatment is initiated, and the patient’s condition improves.  However, he now has fever, headache, and eye pain.  Examination of the nasal cavity shows a black necrotic eschar adherent to the inferior turbinate.  Which of the following procedures would most likely reveal the pathogen responsible for this patient’s infection?

A 24-year-old woman comes to the office due to copious green vaginal discharge and burning with urination for 3 days.  The patient had a yeast infection a year ago, which was treated with over-the-counter medication.  She has recently become sexually active with a new partner and occasionally uses condoms.  Physical examination shows a yellow-green, frothy vaginal discharge and diffuse vaginal erythema but no lesions.  The uterus is anteverted with no cervical motion tenderness.  Which of the following is the best test to confirm the diagnosis in this patient?

A 43-year-old man undergoes induction chemotherapy for acute myeloid leukemia.  Three weeks later, he is hospitalized with fever, cough, and generalized weakness.  On physical examination, his temperature is 38.1 C (100.7 F), blood pressure is 102/68 mm Hg, and pulse is 125/min.  The patient’s pulse oximetry shows 95% on room air.  Crackles are heard over the right upper lung.  His hemoglobin level is 9.5 g/dL, and leukocyte count is 900/mm³ with an absolute neutrophil count of 100/mm³.  Chest x-ray reveals a dense infiltrate involving the right upper lung lobe.  Broad-spectrum antibiotic therapy is instituted.  Five days later, the patient is still febrile.  Bronchoscopy with biopsy is performed and the tissue samples from the bronchoscopy grow mold.  Which of the following organisms is most likely responsible for this patient’s current condition?

A 47-year-old man comes to the office due to persistent fever, night sweats, and fatigue.  Thorough evaluation yields a diagnosis of chronic myeloid leukemia.  While undergoing treatment for his malignancy, the patient experiences headaches, scant nasal discharge, and left eye symptoms.  Physical examination reveals tenderness over the paranasal sinuses and left-sided orbital swelling and cellulitis.  Mild proptosis and ptosis of the left eye are also present.  Biopsy of the sinus mucosa is shown in the image below:

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

A 5-year-old girl is brought to the office by her mother due to 2 weeks of anorexia, nausea, epigastric discomfort, and loose bowel movements.  She has had no fever or bloody stools.  The patient recently returned from rural Brazil, where she spent the summer with family.  She had no gastrointestinal symptoms during the trip but developed intensely pruritic eruptions between the toes of her right foot.  The skin rash spontaneously resolved within a few days and was attributed to insect bites from walking barefoot in the fields.  The patient has no prior medical conditions, takes no medications, and has received all age-appropriate vaccinations.  On physical examination, the abdomen is soft and nontender with normoactive bowel sounds.  Stool microscopy reveals smooth, thin-walled eggs.  If left untreated, this patient’s condition can lead to which of the following complications?

A 38-year-old man is undergoing treatment for acute myeloid leukemia.  Initially, he had fevers and reported being lethargic and fatigued all the time.  However, of late, he has noted increasingly severe right-sided headaches.  He describes the pain to be throbbing in nature and located behind his right eye.  He also has associated nasal stuffiness.  The patient has no prior history of allergies.  Physical examination reveals right-sided proptosis and periorbital tenderness.  Biopsy of the right maxillary sinus mucosa reveals the following:

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

A 16-year-old previously healthy adolescent developed nonproductive cough, dyspnea, and wheezing after traveling abroad.  The patient has no history of allergies and has had no recent upper respiratory illness.  The symptoms spontaneously resolved after several days, but soon after, she began having abdominal discomfort, anorexia, and nausea.  Stool microscopy results are shown in the following image:

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

A 19-year-old man comes to the physician due to diarrhea for the last 4 months.  He visited a student health clinic at his university a month ago and was prescribed a course of antibiotics, but his diarrhea failed to improve.  He also has persistent nausea, a bloating sensation, and a 6.8-kg (15-lb) weight loss.  Microscopic examination of his stool for ova and parasites following trichrome staining is shown in the exhibit.  Impairment of which of the following immune functions would most likely predispose to this patient’s infection?

A 65-year-old man comes to the emergency department due to fevers and malaise for the last 48 hours.  The patient initially experienced fevers and chills, which subsequently progressed to fatigue to the point that he had difficulty performing routine tasks this morning.  He has also had a mild headache.  The patient lives in eastern Massachusetts with his wife.  He is retired and spends time gardening and swimming at a local public pool.  The patient traveled to Thailand 2 years ago with his family and to Florida 2 months ago.  Temperature is 38.9 C (102 F), blood pressure is 122/70 mm Hg, and pulse is 114/min and regular.  The oropharynx is dry and has no lesions.  Lung examination demonstrates coarse crackles bilaterally.  The abdomen is soft and mildly tender throughout with palpable splenomegaly.  He has no rash or open wounds.  Peripheral blood smear is shown in the exhibit.  Which of the following is the most likely source of this patient’s infection?

A 54-year-old man comes to the emergency department due to intermittent fever, headache, body aches, and fatigue for the past week.  He has taken over-the-counter flu medicines, but the symptoms continue to persist.  The patient is a petroleum engineer; 6 months ago, he spent 8 weeks in Brazil developing a new oil field.  Temperature is 37.6 C (99.7 F), blood pressure is 128/72 mm Hg, pulse is 102/min, and respirations are 18/min.  On physical examination, the lungs are clear to auscultation and there are no cardiac murmurs.  The spleen is palpable 2 cm below the left costal margin.  The remainder of the examination is unremarkable.  Laboratory results are as follows:

Peripheral blood smear is shown in the exhibit.  Which of the following is the most likely underlying cause of this patient’s current condition?

A 52-year-old woman is evaluated due to 2 weeks of fever, fatigue, nonproductive cough, and dyspnea, which began shortly after visiting her family in Ohio.  She also lost 1.5 kg (3.3 lb) over the same period.  The patient has a history of rheumatoid arthritis and has been taking adalimumab for the past 6 months.  She had negative tuberculosis skin testing prior to beginning the drug.  Temperature is 38 C (100.4 F), blood pressure is 130/70 mm Hg, pulse is 92/min, and respirations are 18/min.  Physical examination reveals bilateral lung crackles, mild generalized lymphadenopathy, and hepatosplenomegaly.  Chest x-ray shows bilateral nodular densities and hilar lymphadenopathy.  Urine testing is positive for a fungal antigen.  Which of the following pathogenic processes is most important during the development of this patient’s infection?

A 31-year-old man is brought to the emergency department after a tonic-clonic seizure.  The patient has no known medical conditions.  He grew up in a rural part of Guatemala and immigrated to the United States 3 years ago.  The patient works as an architect and owns a cat.  He has no history of tuberculosis.  On examination, the patient is afebrile and has no focal neurologic deficits or meningeal signs.  HIV antibody test and interferon-gamma release assay are negative.  Chest x-ray is normal.  MRI of the head reveals a 1.5-cm cyst within the left sylvian fissure that has minimal enhancement and no associated edema.  Which of the following is the most likely means of acquisition of the infection responsible for this patient’s findings?

A 28-year-old woman is brought to the emergency department due to abrupt onset of high fever, severe headache, photophobia, and confusion.  Temperature is 38.9 C (102 F), blood pressure is 90/60 mm Hg, and pulse is 118/min.  On examination, the patient is obtunded and diaphoretic.  There is increased resistance to passive neck flexion.  Pupillary reflexes are brisk and symmetric, and the patient moves all extremities equally to painful stimuli.  Lumbar puncture is performed, and cerebrospinal fluid (CSF) analysis reveals neutrophilic pleocytosis, elevated protein, and low glucose.  CSF Gram stain shows no bacteria, and fungal antigen testing is negative.  Wet mount preparation of the CSF shows motile trophozoites.  Which of the following most likely predisposed this patient to her current condition?

A 48-year-old previously healthy man is evaluated for progressive fatigue, exertional dyspnea, and excessive bruising.  Physical examination reveals mucosal pallor and scattered ecchymoses.  Blood cell count shows anemia, thrombocytopenia, and leukocytosis with numerous circulating myeloblasts.  Bone marrow biopsy confirms acute myeloid leukemia.  The patient receives induction chemotherapy.  Four days after completion of chemotherapy, the patient develops fever and malaise.  New clusters of macular skin lesions are noted.  Complete blood count shows severe neutropenia.  Blood cultures grow smooth, creamy white, glistening colonies of small yeast forms.  Incubating the pathogen in human serum at 37 C for 2-3 hours leads to formation of small projections from the cell surface with no constriction, as shown in the exhibit.  Which of the following is the most likely organism responsible for this patient’s condition?

A 31-year-old man comes to the office due to oral lesions that cause occasional discomfort during eating.  He also reports fatigue and unintentional weight loss.  The patient has no prior medical problems and takes no medications.  Oral examination findings are shown below.

The lesions can be easily scraped off, revealing erythematous mucosa underneath.  There are several enlarged cervical and axillary lymph nodes. Which of the following diagnostic tests is most appropriate during further workup of this patient?

A neonate born to a 26-year-old woman is evaluated in the newborn nursery.  The patient was delivered vaginally at 38 weeks gestation, and Apgar scores were 8 and 9 at 1 and 5 minutes, respectively.  Initial evaluation shows hydrocephalus, jaundice, and hepatosplenomegaly.  Funduscopic examination reveals retinal exudates and scarring.  Histopathologic evaluation of the placenta shows infiltration of lymphocytes, plasma cells, and macrophages, as well as many areas of calcification.  Within the placental tissue, there are numerous intracellular, crescent-shaped organisms with a central nucleus.  Which of the following maternal interventions would have been most effective in preventing this patient’s condition?

A 24-year-old woman comes to the emergency department due to fevers and chills over the last 24 hours.  Three weeks ago, she underwent a partial bowel resection for Crohn disease without complications.  Since the surgery, the patient has had difficulty eating and has a peripherally inserted central venous catheter in place through which she is receiving parenteral nutrition.  Her temperature is 38.2 C (101 F), blood pressure is 100/60 mm Hg, and pulse is 114/min.  The surgical wound healed well, and her abdomen is soft and nontender.  The catheter site on the left arm is mildly tender but shows no erythema.  Chest x-ray reveals a properly positioned central venous line and is otherwise unremarkable.  Admission blood cultures subsequently return positive for yeast.  The organism responsible for this patient’s bloodstream infection is most likely to exhibit which of the following characteristics?