A diarrheal outbreak is reported at a private school in Columbus, Ohio.  Six healthy children age 10-11 and two teachers developed acute vomiting and diarrhea within a 2-day period.  They describe the diarrhea as watery and without blood or mucus.  Three of those affected are febrile during their illness.  None of the patients have traveled abroad recently, and all are up to date with their vaccinations.  Stool test results are pending.  Which of the following pathogens is the most likely cause of the illness?

A 25-year-old primigravida has a stillbirth at 18 weeks gestation.  Her only symptom during pregnancy was pain in both knees and feet, which she attributed to pregnancy-related weight gain and being “on my feet all day” as an elementary school teacher.  The pain lasted approximately a week and resolved without medication.  The patient was taking prenatal vitamins daily, and her prenatal care was appropriate.  Fetal autopsy shows pleural effusion, pulmonary hypoplasia, and ascites.  Infection with which of the following is the most likely etiology of the stillbirth?

Researchers develop a novel drug to treat HIV infection.  In an in vitro experiment, wild-type viral isolates are cultured with human CD4 T lymphocytes in the presence of the drug.  Microscopic evaluation of these cells reveals no cytopathic changes, and no intracellular viral particles can be detected.  Further analysis reveals that in the presence of the drug the virus attaches to the cellular membrane, but fusion of the viral and cell membranes does not occur.  Which of the following is the most likely target of this medication?

Several individuals in a Florida neighborhood develop a febrile illness during the summer.  Most of them experience abrupt-onset fever, headache, confusion, and lethargy.  Some patients also develop tremors, seizures, and focal motor or sensory deficits.  Physical examination shows signs of meningeal irritation and altered mental status.  Cerebrospinal fluid evaluation reveals lymphocytic pleocytosis with no organisms seen on Gram stain.  Brain imaging shows no abnormalities.  Which of the following would be most helpful in controlling this disease outbreak?

A 9-year-old boy is brought to the clinic by his mother for evaluation of rash and sore throat.  Two days ago, the patient returned home from summer camp with a fever, sore throat, and rash on his extremities.  The fever has resolved, but the rash has continued to spread.  The patient has no chronic medical conditions and has received all routine vaccinations.  Vital signs are normal.  Oropharyngeal examination shows superficial ulcers on the buccal mucosa and soft palate but no tonsillar enlargement or exudates.  Skin examination is shown in the exhibit.  The infectious agent most likely responsible for this patient’s condition is also associated with which of the following complications?

A virologist is researching acyclovir-resistant herpes simplex virus (HSV).  Viral DNA analysis reveals a mutation in the thymidine kinase gene, resulting in a mutated enzyme that does not phosphorylate acyclovir to its active form.  In an experiment, a resistant HSV type 2 strain containing this mutation is cultured in a cell line, and the cell culture is coinfected with a nonresistant HSV type 1 strain.  It is found that some of the newly produced type 1 virions acquire resistance to acyclovir, and subsequent progeny continue to be resistant.  Which of the following mechanisms best explains the observed findings?

A 2-month-old girl is brought to the clinic due to perinatal hepatitis B exposure.  Her mother is age 22 and immigrated to the United States shortly after giving birth.  The pregnancy was complicated by maternal hepatitis B infection.  However, the mother received no prenatal or perinatal care and was not diagnosed with chronic hepatitis B until the immigration process.  Maternal HBsAg, HBeAg, and anti-HBc are all positive; anti-HBs is negative.  The infant has not been evaluated previously and has not received any immunizations or other medications.  She is exclusively breast fed and has been growing normally.  Vital signs are normal and physical examination is unremarkable.  No hepatomegaly or jaundice is seen.  The infant’s laboratory results will most likely show which of the following?

A 38-year-old woman who works as a nurse at a large community hospital is undergoing evaluation following an accidental needlestick in the operating room.  The patient being operated on is not known to have any infections.  As part of her evaluation, the nurse has serologic testing for viral hepatitis.  Laboratory results are as follows:

Which of the following statements is most likely to be true regarding the nurse’s viral status?

A 9-month-old girl is brought to the emergency department after a 2-minute generalized seizure.  The patient has had a fever for the past 12 hours and has been very fussy.  She takes no medications and has no allergies.  Her temperature is 39.4 C (103 F).  Examination shows no abnormalities, and the patient is discharged home.  Three days later, she is afebrile but develops a maculopapular rash on her trunk.  Infection with which of the following is the most likely cause of this patient’s condition?

A 4-year-old boy is brought to the office due to high fever, nasal discharge, and cough for 3 days.  His family recently returned from a vacation in Mexico.  Physical examination shows an ill-appearing, febrile child.  The patient has bilateral conjunctival injection and several small white spots with an erythematous base on his buccal mucosa.  A review of the medical record shows parental refusal of preventive health services in favor of a “natural” approach to the child’s health.  Over the next several days, which of the following is most likely to develop?

A 7-year-old girl is brought to the office due to a diffuse rash.  The father says she developed a lace-like, erythematous rash this morning on her stomach, back, and extremities.  Two days ago, the girl developed red, flushed cheeks.  She also had congestion, headache, and a low-grade fever for 3 days earlier this week.  Last week, her younger brother had similar symptoms that resolved spontaneously.  The infectious agent most likely responsible for this patient’s disease replicates in progenitors of which of the following cell lines?

A 41-year-old woman comes to the emergency department due to 2 days of severe shortness of breath, high fevers, cough, and myalgias.  The patient is a businesswoman who works in the poultry industry.  She recently returned from a trip to China, where she visited several factories in different parts of the country.  Her temperature is 39.1 C (102.4 F).  The patient is in respiratory distress, and crackles and diffuse wheezes are present on pulmonary examination.  Laboratory results show that she contracted an orthomyxovirus that was responsible for an upper respiratory infection epidemic in China.  Transmission of the virus was originally thought to occur only through contact with poultry, but during the outbreak, sustained human-to-human transmission was observed.  Which of the following is most likely responsible for the infectivity of this virus in humans?

During the course of a week at an overnight summer camp, 4 children age 7-9 are sent to the camp health center.  They each have fever, cough, congestion, sore throat, and red eyes.  Physical examination of the children shows bilateral conjunctival injection and an erythematous oropharynx.  Auscultation of one child also reveals crackles in the left lower lung field.  All the children’s symptoms improve over 7 days with supportive care.  Which of the following is the most likely cause of the outbreak?

A 34-year-old man came to the office due to white plaques on his buccal mucosa that he incidentally discovered while brushing his teeth.  After appropriate workup, he was found to be HIV positive, with a CD4+ T lymphocyte count of 280/mm³.  He was referred to an infectious diseases specialist for follow-up but was noncompliant with appointments.  The patient returns 3 years later due to several months of pain and itching in the perirectal area.  He also has intermittent rectal bleeding and often sees bright red blood on the tissue after wiping.  The patient did not seek medical care because he thought he had hemorrhoids.  On examination, a single, hard mass with superficial ulceration measuring approximately 2×2 cm is noted in the anal canal.  No hemorrhoids are present.  There is no palpable regional lymphadenopathy.  Which of the following pathogens is most likely responsible for this patient’s current anal pathology?

Researchers investigating infections in immunocompromised individuals enroll patients with solid-organ malignancy who have received conventional cytotoxic chemotherapy.  Detailed microbiologic data are obtained on any patient who develops fever during periods of chemotherapy-induced neutropenia.  Which of the following pathogens are most likely to be identified as the cause of these patients’ infections?

Researchers find that even though the current recombinant hepatitis B vaccine is immunogenic, about 10% of immunocompetent individuals do not develop high enough levels of anti-HB antibodies to confer immunity despite receiving the recommended 3 doses of the vaccine.  To increase the vaccine immunogenicity, they use a lipopolysaccharide derivative as a vaccine adjuvant.  The new vaccine induces an exaggerated immune response in previous nonresponders, which results in high antibody titers and, possibly, lifelong protection.  Universal vaccination with the new vaccine would most likely lead to eradication of which of the following conditions?

Researchers are studying mechanisms of human infection by animal viruses.  The investigators induce random mutations in the genome of an avian influenza virus that is unable to infect humans but is structurally similar to human influenza A virus.  A mutated isolate is found to be able to infect human upper respiratory tract epithelial cells.  Alteration in which of the following viral components most likely enabled this novel strain to cause cross-species infection?

A 24-year-old woman comes to the office due to 3 weeks of worsening throat pain and difficulty swallowing.  She has no chronic medical problems but has had recurrent vulvovaginal candidiasis and bacterial folliculitis over the past year.  The patient has also had an unintentional weight loss of 4.5-kg (10-lb) over the past 3 months.  She is an exchange student from Senegal.  Physical examination shows generalized lymphadenopathy and oropharyngeal thrush.  Upper endoscopy confirms Candida esophagitis.  HIV antigen/antibody immunoassay returns positive.  Further testing shows a CD4 count of 180/mm³ but there is no detectable plasma HIV-1 RNA.  Which of the following best explains the observed laboratory findings in this patient?

A 36-year-old man comes to the office due to 2 months of fatigue, intermittent arthralgias, and poor appetite.  The patient has no prior medical conditions and takes no medications.  He does not use tobacco or illicit drugs, and he drinks alcohol occasionally.  The patient has had multiple sexual partners and uses condoms inconsistently.  Physical examination shows no abnormalities.  Liver aminotransferases are elevated.  Hepatitis serology shows the following:

Which of the following genome replicative processes is most likely used by the virus infecting this patient?

A 25-year-old man is hospitalized with fever, anorexia, and fatigue.  He admits to IV drug abuse and many episodes of unprotected sex over the past few years.  His condition is treated and he is discharged from the hospital.  The patient follows up with his physician regularly over the next 8 months, and the following serologic marker dynamics are obtained (see the graph below).

*Grey bars indicate the detectable presence of the specified serum marker.

This patient most likely:

An unvaccinated 20-month-old girl is brought to the emergency department with a rash.  Three days ago, she developed a fever along with cough, congestion, and red eyes.  The rash appeared on her face yesterday and spread to her trunk, arms, and legs today.  The patient’s temperature is 39.4 C (103 F).  She is lethargic and ill appearing.  Physical examination shows conjunctival injection and a diffuse, maculopapular, erythematous rash.  She is admitted to the hospital for further management.  Deficiency of which of the following is associated with a high rate of complications from this patient’s condition?

A 35-year-old man is found to have elevated alanine aminotransferase and aspartate aminotransferase levels.  He has no abdominal pain, nausea, or vomiting.  The patient has used intravenous drugs in the past but denies significant alcohol use.  His past medical history is negative for blood transfusions or excessive acetaminophen use.  Physical examination findings are normal.  Laboratory studies show high titers of IgG directed against the hepatitis C envelope protein.  Which of the following is the most likely reason that these antibodies do not confer effective immunity against the infection?

A 22-year-old man who recently immigrated to the United States comes to the physician for routine care.  He has no current symptoms.  He takes no medications and has no known allergies.  Laboratory testing yields the following results:

The patient’s country of origin does not offer universal immunization against hepatitis A, and he says he has not been vaccinated against the virus.  Which of the following do these laboratory findings most likely suggest?

A 34-year-old man comes to the physician due to fever, myalgias, malaise, and progressive fatigue over a 2-week period.  He has not experienced any sore throat.  He recently received a blood transfusion while hospitalized for a bleeding duodenal ulcer.  Physical examination shows mild splenomegaly.  There is no lymphadenopathy and no jaundice.  Lymphocytosis is identified in the peripheral blood with 30% atypical lymphocytes.  The patient’s serum fails to agglutinate horse erythrocytes.  The agglutination test is repeated a week later and, again, no agglutination is noted.  HIV testing is negative.  Which of the following is the most likely cause of this patient’s condition?

A 23-year-old pregnant woman is found to have chronic viral hepatitis B on liver biopsy.  The presence of which of the following serologic markers is most likely to increase the risk of vertical transmission of the virus?

An outbreak of hepatitis in New Delhi was characterized by high incidence of fulminant hepatitis in pregnant women, who experienced a mortality rate of 20%.  The virus responsible for the outbreak:

A 23-year-old man comes to the emergency department due to 7 days of fever and malaise.  He has also had nausea and a poor appetite.  The patient has no significant medical history but regularly uses injection drugs and has had multiple sexual partners.  Temperature is 38 C (100.4 F).  On physical examination, there is scleral icterus.  A tender, smooth liver edge is palpable 4 cm below the right costal margin.  Appropriate therapy is initiated.  The following graph illustrates this patient’s hepatitis B serologic markers over time.

Which of the following best explains these observed findings?

A 3-year-old girl is brought to the emergency department with abrupt-onset vomiting followed by frequent, large-volume, watery diarrhea for the last day.  She has no prior medical conditions but has not received recommended vaccinations.  Temperature is 37.8 C (100 F).  Physical examination shows mild dehydration.  The abdomen is soft and mildly tender to palpation throughout.  Bowel sounds are increased.  Polymerase chain reaction testing of the stool sample yields a virus with a segmented, double-stranded RNA genome.  Which of the following pathologic findings is most likely to be present in this patient?

A 29-year-old male is hospitalized with low-grade fever and high serum levels of alanine aminotransferase (ALT) and aspartate aminotransferase (AST).  His serum is positive for HBsAg.  You should inform the patient that the most likely outcome for his infection would be:

A 45-year-old woman with HIV comes to the office due to the appearance of multiple tender nodules on her lower extremities over the past several weeks.  The patient tried to squeeze open the lesions, but this led to profuse bleeding with no expression of pus.  She works at a pet store in Mississippi and has regular contact with a variety of animals.  Skin examination findings are shown below.

Biopsy of the lesions reveals large endothelial cells forming vascular channels with a mixed neutrophilic and lymphocytic infiltrate.  Immunohistochemical staining for human herpesvirus 8 is negative.  Infection with which of the following pathogens is most likely responsible for

A 23-year-old woman participates in a research study evaluating the effect of monoclonal antibodies on viral infection.  The patient has no past medical history and takes no medications.  She has not been ill recently.  During the study, a peripheral blood specimen is obtained.  CD19 positive cells are isolated, purified, and exposed to monoclonal antibodies against cell surface complement receptor CD21.  The cells are subsequently incubated along with several viruses being studied.  Initial exposure to monoclonal antibodies against CD21 is most likely to prevent cell infection with which of the following viruses?

A 20-year-old woman comes to the urgent care center due to pain with intercourse and dysuria for the last 3 days.  She also has had low-grade fevers and a mild headache.  The patient is otherwise healthy and takes oral contraceptives only.  She has no history of urinary tract or sexually transmitted infections.  The patient is sexually active and recently began a relationship with a new partner.  Temperature is 37.2 C (99 F).  The abdomen is soft and nontender.  Pelvic examination shows multiple, painful, shallow ulcers with an erythematous base on the left labium.  No vaginal discharge is present.  Bimanual examination reveals no cervical motion tenderness.  There is bilateral tender inguinal lymphadenopathy.  Which of the following is the best diagnostic test for this patient’s condition?

A 1-month-old boy is brought to the emergency department during winter with cough and difficulty breathing.  He has had rhinorrhea, cough, and congestion for the past 3 days.  Today, the patient began breathing harder and faster with increased cough.  Temperature is 37.8 C (100 F) and respirations are 68/min.  Oxygen saturation is 91%.  The patient is alert and awake and has subcostal and suprasternal retractions.  Mucous membranes are moist, and the anterior fontanelle is open and flat.  Physical examination reveals diffuse expiratory wheezes and scattered rales.  Which of the following pathogens is most likely causing this patient’s condition?

A 21-year-old woman volunteers in a research study evaluating the effects of viral infection on immunoglobulin production.  The patient is a college student who has no past medical history and does not take any medications.  She has not felt ill recently.  Her lymphocytes are isolated from whole blood and cultured.  The mature B lymphocytes exposed to a specific virus in vitro acquire the ability to proliferate indefinitely while maintaining the capacity to secrete immunoglobulins.  Which of the following tests is used to diagnose an in vivo infection with this virus?

Studies conducted following an influenza epidemic that affected the inhabitants of several countries determine that the cause is an antigenically novel virus strain carrying the animal-strain hemagglutinin and neuraminidase surface molecules.  Further analysis reveals that reassortment of the genetic materials occurred during coinfection with human influenza virus and swine influenza virus, leading to the development of some progeny that contain genetic materials from both viruses.  Which of the following viruses can undergo a similar process?

A 29-year-old woman is brought to the emergency department with new-onset generalized tonic-clonic seizures.  She has a history of HIV and has not been taking her medications consistently.  According to her family, she has been sleepy and weak for the past 2 weeks.  She has not had much of an appetite and has recently spent most of her time in bed.  MRI of the head demonstrates 2 ring-enhancing focal lesions with surrounding edema in the right frontal lobe.

Item 1 of 2

Which of the following would be effective against the pathogen most likely responsible for this patient’s symptoms?

Item 2 of 2

After 10 days of empiric therapy, the patient’s clinical situation has not improved significantly and the appearance of the brain lesions on MRI is unchanged.  Results for Toxoplasma serology and polymerase chain reaction (PCR) testing are negative.  Cerebrospinal fluid PCR analysis for Epstein-Barr virus is positive.  Tissue biopsy from the frontal lobe lesions would likely predominantly show an abnormal population of which of the following cells?

A 42-year-old man with HIV comes to the emergency department with 3 days of abdominal pain, decreased appetite, and diarrhea.  Temperature is 38 C (100.4 F), blood pressure is 98/60 mm Hg, and pulse is 110/min.  Abdominal examination shows tenderness over the left lower quadrant without guarding or rebound tenderness.  CD4 count is 38/mm³.  Colonoscopy shows erythematous, friable mucosa with superficial ulcerations in the sigmoid and descending colon.  Biopsy from the ulcers reveals inflammatory infiltrate, mucosal necrosis, and enlarged endothelial cells with large, ovoid nuclei containing prominent basophilic deposits.

Item 1 of 2

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

Item 2 of 2

The patient is hospitalized, and appropriate treatment is begun.  After discussing the underlying cause of his condition—immunosuppression due to HIV infection—the patient agrees to take antiretroviral therapy consistently.  One of the medications in his treatment regimen leads to the production of immature, noninfectious virions containing large polyproteins.  Which of the following viral processes is most likely directly inhibited by this agent?