A 17-year-old boy is hospitalized with recent-onset insomnia, headaches, periodic agitation, and dysphagia.  He is a high school student interested in becoming a veterinarian.  He owns a snake, fish, and parrot.  About 4 weeks ago, went on a field trip to study bats in a cave.  The patient has no known medical problems and is not taking any medications.  He does not consume alcohol and has never used illicit drugs.  Despite hospitalization and treatment, he develops painful spasms, progressive paralysis, and coma and dies.  Which of the following interventions would have most likely prevented this student from developing this illness?

A 7-year-old boy is brought to the emergency department due to fever, headache, vomiting, and confusion.  He has no prior medical conditions and is up to date with immunizations.  The patient has not traveled recently, and his parents report no exposure to individuals with similar symptoms or ingestion of anything out of the ordinary.  They recently noticed several dead birds around their neighborhood.  Temperature is 39 C (102.2 F).  Physical examination shows neck rigidity and right lower extremity weakness with fasciculations.  Cerebrospinal fluid analysis shows a leukocyte count of 200/mm³ with 20% neutrophils and 80% lymphocytes.  Which of the following pathogens is most likely responsible for this patient’s condition?

A previously healthy 2-year-old boy is brought to the clinic with fever and mouth pain that began yesterday.  He has consumed an adequate amount of fluids but refuses to eat due to the pain.  The patient has no medical problems and takes no medications.  Physical examination reveals swollen gums and vesicular, inflamed lesions on his hard palate and lips.  He has enlarged and tender cervical lymph nodes.  Which of the following is most likely responsible for this patient’s condition?

A 29-year-old woman comes to the emergency department due to high fever and a rash.  The illness began 5 days ago with acute-onset high fever, headache, retroorbital pain, and joint and muscle pain.  Yesterday, the patient developed a diffuse rash, and this morning she had a nosebleed.  The patient returned from a trip to Brazil 10 days ago.  Her boyfriend traveled with her and is asymptomatic.  Examination shows a diffuse maculopapular rash and scattered petechiae over her extremities.  Laboratory evaluation reveals thrombocytopenia.  Transmission of the pathogen causing this patient’s current symptoms occurs in a manner that is most similar to which of the following diseases?

A 45-year-old woman is hospitalized due to severe Pneumocystis pneumonia.  The patient has a history of injection drug use and was diagnosed with HIV 10 years ago.  She does not adhere to antiretroviral therapy and has developed resistance to multiple antiretroviral medications.  Laboratory evaluation reveals severe immunosuppression with a high HIV viral load.  After the patient is adequately treated for pneumonia, HIV genotypic analysis is performed.  The viral isolates are found to have a mutation in the variable (V3) region of the env gene, which makes the virus capable of infecting T cells with a specific surface receptor, but infection of macrophages is inefficient.  Which of the following antiretroviral medications is most likely to be ineffective in this patient?

A 34-year-old man is evaluated due to elevated liver aminotransferases.  The patient has no chronic medical conditions but has a history of injection drug use.  Family history is notable for liver cirrhosis in his mother.  Ultrasonography-guided liver biopsy is performed.  Histopathology demonstrates marked panlobular mononuclear cell infiltration that spans adjacent lobules.  Intensely eosinophilic round bodies are occasionally seen scattered among the hepatic parenchyma.  Which of the following is the most likely cause of the latter histopathologic finding in this patient?

A 32-year-old male with anorexia and fatigue is found to have persistently elevated serum alanine aminotransferase (ALT) and aspartate aminotransferase (AST) levels.  He is also positive for HBsAg.  Which of the following best describes the mechanism of liver injury in this patient?

A 39-year-old man comes to the clinic with a month-long history of fever, fatigue, joint pain, and an urticarial skin rash.  He did not seek attention immediately as he hoped his symptoms would resolve.  The patient is generally improving but is still very easily fatigued.  Past medical history is unremarkable and the patient takes no regular medications, although he has not seen a physician since early childhood.  He is not married and has a history of multiple heterosexual partners.  The patient recently stopped smoking, drinks alcohol socially and does not use illicit drugs.  Physical examination is notable for a moderately enlarged and tender liver.  No jaundice is seen.  Laboratory studies show significant elevations in serum alanine aminotransferase (ALT) and aspartate aminotransferase (AST).  Which of the following is the most likely diagnosis in this patient?

A 34-year-old man comes to the office due to anorexia, nausea, and dark-colored urine for the last 2 days.  He travels frequently for business and recently returned from a trip to Mexico.  The patient has a history of asthma and occasionally uses inhaled albuterol for symptom management.  Temperature is 37.8 C (100.2 F), blood pressure is 125/70 mm Hg, and pulse is 94/min.  Cardiopulmonary examination is normal.  There is mild right upper quadrant abdominal tenderness with no guarding or rebound.  Serologic testing for Entamoeba histolytica is negative.  Which of the following histologic findings is most likely to be found within this patient’s liver?

A 32-year-old man comes to the office due to fever, jaundice, vomiting, and decreased appetite for the past 5 days.  He recently returned home from South America and reports that a couple of his travel companions have had similar symptoms.  The patient does not use alcohol, tobacco, or illicit drugs and has no relevant family history.  Temperature is 38.8 C (101.8 F), blood pressure is 125/80 mm Hg, pulse is 85/min.  Scleral icterus and hepatomegaly are present.  Laboratory results are as follows:

Which of the following pathophysiologic events is most likely occurring in this patient?

A 36-year-old man comes to the office for follow-up regarding hepatitis C.  The patient was last seen 3 years ago for a 2-week history of fatigue, nausea, and decreased appetite; at that time, he was diagnosed with hepatitis C infection due to injection drug use.  The patient was offered treatment but declined.  He is currently healthy and feels well.  There is no family history of liver disease.  The patient currently does not use illicit drugs or alcohol.  Vital signs are normal.  BMI is 26 kg/m².  Examination is normal.  Laboratory results are as follows:

Liver ultrasound is negative for cirrhosis or masses.  If a liver biopsy is performed, which of the following histologic findings would most likely be present in this patient?

A 46-year-old man experiences burning pain in his left thigh and leg.  The next day, he notices groups of erythematous, tender, fluid-filled vesicles in the painful region.  The lesions gradually crust over the next several days as shown in the exhibit.  Which of the following dermatomes are most likely involved in this patient’s disease process?

A 46-year-old woman comes to the office to be tested for HIV after one of her prior sexual partners found that he was HIV-positive.  She feels healthy and has no symptoms.  The patient has never had a sexually transmitted infection and has no chronic medical conditions.  She has had several sexual partners in her lifetime and uses condoms inconsistently.  Physical examination shows no abnormalities.  HIV antigen/antibody immunoassay and a confirmatory HIV-1 antibody test are positive.  Further studies reveal plasma HIV viral load is >250,000 copies/mL and CD4 lymphocyte count is 150 cells/mm3.  After discussing the results and need for treatment, the patient agrees to start antiretroviral therapy.  An additional medication to prevent opportunistic infections is also planned.  This added treatment is most likely being used to prevent infection from which of the following pathogens?

A previously healthy 27-year-old man comes to the clinic due to fever and rash.  The patient is a missionary who recently immigrated from South Africa with his wife and 3 young children.  Four days ago, he developed a high fever and malaise followed by the appearance of multiple discrete macules on his face, trunk, and extremities.  The lesions are pruritic and evolved into pustules then vesicles.  The patient has continued to develop new vesicles as other lesions crust over and heal.  The remainder of the examination is unremarkable.  This patient’s symptoms are most likely due to an infection with which of the following pathogens?

A 31-year-old man comes to the office due to a week of fever, night sweats, sore throat, malaise, and extreme fatigue.  He has no prior medical conditions and takes no medications.  The patient has had multiple sexual partners over his lifetime and uses condoms inconsistently.  Temperature is 38 C (100.4 F), blood pressure is 110/80 mm Hg, pulse is 92/min, and respirations are 20/min.  Physical examination shows multiple shallow ulcers on the oropharyngeal mucosa, and the tonsils are enlarged and hyperemic.  Cervical and axillary lymphadenopathy is present, and there is a generalized, erythematous, maculopapular skin rash.  HIV testing shows the following results:

Which of the following best explains this patient’s laboratory findings?

A 22-year-old woman comes to the office for a routine prenatal visit.  She is 16 weeks pregnant with her first child.  Two months ago, she had a mononucleosis-like illness with fever, myalgia, and fatigue, and she was diagnosed with cytomegalovirus infection.  The patient recovered with symptomatic treatment and is currently asymptomatic.  Her other medical problems include episodic migraine headaches and benign choroidal nevus.  She takes prenatal vitamins and does not use tobacco, alcohol, or illicit drugs.  The patient has a family history of hypertension and glaucoma.  Physical examination findings and prenatal ultrasound are unremarkable.  This patient’s unborn infant is at greatest risk for which of the following eye conditions?

An 85-year-old woman comes to the emergency department due to worsening eye pain.  The patient noticed a rash around the left eye yesterday and, this morning, began experiencing left eye pain, redness, and decreased vision.  Physical examination shows scattered vesicles on the left side of the forehead and nose and on the left upper and lower eyelids.  Left eye examination shows conjunctival erythema, and fluorescein staining demonstrates linear, branching ulcerations on the cornea.  Right eye examination and the remainder of the physical examination are normal.  Which of the following pathogens is most likely responsible for this patient’s current condition?

A 27-year-old man comes to the emergency department due to 2 weeks of low-grade fever, malaise, and loss of appetite.  The patient has no prior medical conditions and takes no medications.  He does not smoke cigarettes or drink alcohol but regularly injects illicit drugs.  The patient is sexually active and uses condoms inconsistently.  Physical examination reveals scleral icterus.  A smooth and mildly tender liver edge is palpable 4 cm below the right costal margin.  Laboratory results are as follows:

Which of the following is an essential step in the replication cycle of the virus infecting this patient?

A 43-year-old, HIV-positive woman comes to the office due to lesions on her vulvar and perineal areas for the past 4 weeks.  On examination, there are clusters of firm, shiny, round papules that are each 2-6 mm in diameter with central indentation.  Microscopic examination of a lesion is shown in the image below:

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