A 29-year-old woman comes to the office due to 2 weeks of severe, progressive joint pain in her hands, wrists, and ankles.  The pain started in several fingers and spread over the course of a day to more joints.  It is so severe that she is unable to perform daily activities, and ibuprofen provides only partial relief.  The patient has no medical conditions but developed a high fever and diffuse rash on her trunk and extremities during a trip to Puerto Rico 2 weeks ago.  Evaluation at a local medical facility was negative for malaria, and her fever and rash resolved spontaneously after a few days.  She does not use tobacco, alcohol, or illicit drugs and is in a monogamous relationship with her husband.  Temperature is 37 C (99 F), blood pressure is 122/70 mm Hg, and pulse is 86/min.  On physical examination, the patient has pink, moist oropharyngeal mucosa; no rash; clear lungs; normal heart sounds; and no abdominal organomegaly.  Periarticular swelling and tenderness are noted in the wrists, ankles, and interphalangeal joints of her hands.  Range of motion in these joints is decreased due to pain.  Laboratory testing reveals lymphopenia.  Which of the following is the most likely cause of this patient’s joint pain?

A 4-year-old boy is brought to the office with 3 days of fever, irritability, and oral lesions.  His parents are concerned due to his refusal to eat or drink and lack of urination today.  Physical examination demonstrates painful ulcers on the tongue and gingiva, swollen gums, and cervical lymphadenopathy.  The remainder of his examination is unremarkable.  Microscopic examination of an oral ulcer base scraping is shown in the image below.

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

A 30-year-old man is brought to the emergency department due to erratic behavior, progressive right-sided weakness, and difficulty walking for the last month.  The patient was diagnosed with HIV several years ago and is not compliant with antiretroviral treatment.  Physical examination shows a cachectic and disheveled man.  Funduscopic examination shows no abnormalities.  Motor strength in the right upper and lower extremities is decreased, and his gait is ataxic.  Brain MRI reveals several discrete areas of demyelination in the subcortical and periventricular white matter with no surrounding edema or mass effect.  CD4 cell count is 30/mm³.  Reactivation of which of the following pathogens is most likely responsible for this patient’s current symptoms?

A 23-year-old man comes to the office due to penile ulcers that appeared after unprotected sexual intercourse.  Physical examination shows 3 subcentimeter ulcers on the penile shaft.  Microscopic examination of a scraping from an ulcer base reveals the following:

Which of the following medications is most appropriate for this patient?

A 24-year-old woman, gravida 1 para 0, at 26 weeks gestation comes to the emergency department due to abdominal cramping and vaginal bleeding.  She recently emigrated from Bolivia and reports no prior medical problems.  The patient’s only medication is a daily multivitamin.  Immunization status is unknown.  The patient spontaneously delivers a stillborn fetus with multiple congenital malformations.  Fetal autopsy reveals microcephaly with thin cerebral cortices, ventriculomegaly, and subcortical calcifications.  Viral RNA of a neurotropic virus is detected in body fluids of the mother and the fetus.  Transplacental transmission of this virus causes apoptosis of neural progenitor cells in the developing fetus, leading to disruption of neuronal proliferation, migration, and differentiation.  Which of the following viruses most likely caused this patient’s in utero infection?

A 34-year-old man comes to the emergency department due to 5 days of progressive anorexia, nausea, and abdominal pain.  The patient is a known hepatitis B carrier.  He admits to using IV drugs and has shared needles with other drug users on several occasions.  His temperature is 37.7 C (99.9 F).  Examination shows scleral icterus and mild, tender hepatomegaly.  Laboratory studies are notable for highly elevated levels of liver aminotransferases and serum bilirubin and also show the following results:

This patient’s chronic infection assists which of the following life cycle aspects of the current infectious agent?

A scientist is conducting research to discover novel drug targets for antiretroviral agents against HIV.  An HIV-positive blood sample is analyzed in the laboratory, and purified CD4 T lymphocytes are extracted.  Using molecular tracing techniques, the scientist follows a viral HIV protein that is synthesized in the blood sample’s T cells.  The protein becomes glycosylated before being proteolytically cleaved into 2 smaller proteins in the Golgi apparatus.  Which of the following best describes the function of these 2 newly formed HIV proteins?

The influenza A virus uses a sialic acid as a receptor for cellular entry.  The chemical composition of the specific type of sialic acid that the virus interacts with differs among animal species.  The avian influenza virus attaches to a specific sialic acid that is present in fowl epithelial cells but absent in human cells, whereas the human influenza virus can only attach to the type of sialic acid present in human respiratory epithelium.  When pig epithelial cells, which contain both types of sialic acid, are infected with the combination of avian and human influenza A viruses, some of the new avian influenza viral particles become capable of attaching to human cells.  However, the progeny of this new strain of avian influenza virus is still unable to attach to human epithelial cells.  Which of the following phenomena best describes the change in the avian influenza virus in this experiment?

 14-year-old girl comes to the emergency department due to rapidly progressive exertional dyspnea and generalized weakness, which were preceded by a mild febrile illness several days ago.  She has a history of sickle cell disease and takes a daily folic acid supplement.  Her temperature is 37.2 C (99.1 F), blood pressure is 115/70 mm Hg, and pulse is 112/min and regular.  On examination, the patient has conjunctival pallor but no icterus.  A cardiac flow murmur is present.  The abdomen is soft and nontender with no organomegaly.  Laboratory studies show a hematocrit of 16% with reticulocyte count of 0.1% (normal range, 0.5%-1.5%).  Leukocyte and platelet counts are normal.  Which of the following best describes the virus most likely responsible for this patient’s current condition?

A pharmaceutical researcher is trying to develop a vaccine against the hepatitis C virus.  She infects a chimpanzee with hepatitis C virus of known genotype and subtype.  Several weeks later, a liver sample is obtained, and viral RNAs are extracted from the hepatocytes.  A genetic study of the viral genomes reveals that the extracted RNA sequences vary significantly from that of the original infecting virus.  This genetic instability is most likely due to the lack of which of the following features during the viral replication process?

A 32-year-old man is evaluated due to several weeks of watery diarrhea.  He has also had intermittent, crampy abdominal pain and has lost 7.0 kg (15.4 lb) during this period.  The patient has a history of HIV and was hospitalized 6 months ago for Pneumocystis pneumonia.  On physical examination, the patient is afebrile and the abdomen is soft and nontender with no organomegaly.  Stool tests for leukocytes and occult blood are negative.  Endoscopy is performed and mucosal biopsy shows an inflammatory infiltrate in the lamina propria and pathogens lining the epithelium, as shown in the exhibit.  Which of the following is the most likely cause of this patient’s condition?

A 34-year-old woman comes to the office with recent onset of malaise.  The patient reports feeling “run down and under the weather” but otherwise has no symptoms.  She is an avid cyclist and is concerned about her ability to participate in a charity race in 3 days.  The patient works as a nurse at a local hospital and lives at home with her husband and 2-year-old son.  She occasionally smokes when she goes out, but since she began feeling ill, she no longer has the desire to smoke.  Physical examination is notable for hepatomegaly.  Laboratory results are as follows:

Which of the following is most likely to be elicited on further history taking?

A 46-year-old homeless man comes to the emergency department with fever and chest pain that worsens with swallowing.  The patient has been hospitalized several times recently with Pneumocystis jirovecii pneumonia.  He has a history of intravenous drug use.  His temperature is 37.8 C (100 F).  Oropharyngeal examination is remarkable only for poor dentition.  Esophagogastroduodenoscopy is performed and reveals esophageal hyperemia and linear ulcerations.  Which of the following is the most likely cause of this patient’s condition?

A 26-year-old man is being screened as a potential kidney donor for his mother, who has end-stage renal disease.  The patient has no history of diabetes, hypertension, or kidney disease and is up to date with all recommended vaccinations.  Physical examination is unremarkable.  Blood type and crossmatch testing demonstrate biological compatibility.  As part of the evaluation, screening for transmissible diseases is performed and is notable for the following results:

Based on these data, the patient most likely has had which of the following

conditions in the past?

A 23-year-old man comes to the physician due to a 2-month history of fatigue, malaise, and abdominal discomfort.  He is found to have tender hepatomegaly with elevated liver function tests.  The patient has never been vaccinated against hepatitis.  He has had no raw or uncooked foods recently and recalls no ill contacts.  There is no history of blood transfusion.  The patient is a graduate student who immigrated to the United States 2 years ago and has not traveled outside the country since.  He smokes 2 packs of cigarettes a day and consumes 1 or 2 bottles of beer on weekends.  The patient does not use illicit drugs.  He has had several episodes of unprotected sex with different female partners within the past year.  Which of the following is most likely to be present in this patient?

A 65-year-old man comes to the office after his wife insisted he get a checkup.  The patient feels well and has no chronic medical conditions but has not seen a physician in many years.  He received blood transfusions after a motor vehicle trauma in his 20s.  The patient does not use tobacco or illicit drugs but drinks 2 or 3 alcoholic beverages daily.  Physical examination reveals no hepatomegaly, ascites, or dilation of the superficial abdominal veins.  The remainder of the examination shows no abnormalities.  Serology is positive for hepatitis C antibodies.  Chronic hepatitis C infection is confirmed with positive HCV RNA testing.  After appropriate counseling is provided, combination therapy with sofosbuvir and ledipasvir is planned.  This treatment is most likely to help clear the infection through which of the following mechanisms?

A 34-year-old woman comes to the office due to 3 days of discomfort and swelling in the right axilla.  The patient has no known medical problems.  She is a veterinarian and volunteers as a swim coach at her children’s school.  She owns a parrot and a hamster.  Temperature is 36.9 C (98.4 F).  On physical examination, there is an enlarged, tender axillary lymph node on the right, measuring about 3 cm and with slight surrounding erythema but no skin breakdown.  There is no other area of lymphadenopathy.  Several scratch marks are seen on the right arm.  The organism responsible for the lymphadenopathy is also associated with which of the following conditions?

A 68-year-old woman is brought to the emergency department due to confusion and lower extremity weakness.  Her symptoms began 2 days ago with fever, headache, malaise, and myalgias.  Today, the patient became confused and had difficulty ambulating due to left lower extremity weakness.  The patient has a history of lymphoma in remission, hypertension, and type 2 diabetes mellitus.  She drinks alcohol socially but does not use tobacco or recreational drugs.  She is visiting her family in Texas for the summer and has never traveled outside the United States.  Temperature is 38.3 C (101 F), blood pressure is 130/70 mm Hg, and pulse is 96/min.  The patient is oriented to place and person only and has a coarse hand tremor.  There is flaccid paralysis of the left lower extremity with preserved sensation.  CT scan of the head shows no abnormalities.  Lumbar puncture is performed, and cerebrospinal fluid analysis shows lymphocytic pleocytosis and elevated protein.  Cerebrospinal fluid polymerase chain reaction testing yields viral RNA.  The infectious agent responsible for this patient’s current condition is most likely transmitted via which of the following mechanisms?

A 68-year-old woman comes to the office for evaluation of a skin rash.  The patient says that red, painful papules erupted on her right chest 5 days ago and quickly turned into vesicles containing clear fluid.  The lesions are now beginning to crust and are less painful.  She has never had similar lesions and does not recall coming into contact with a person having a similar illness.  Medical history is significant only for well-controlled hypertension.  Physical examination shows a grouped vesicular rash involving the right T4 dermatome that is beginning to heal with crusting.  The rash does not cross the midline.  Lightly touching the skin elicits sharp pain.  The remainder of the examination shows no abnormalities.  This patient is at greatest risk of developing which of the following complications as a result of her current condition?

A 23-year-old man has 2 days of fever, cough, sore throat, and runny nose.  His temperature is 38 C (100.4 F).  Lung sounds are clear to auscultation.  A nasopharyngeal swab is obtained.  Naked viral particles are seen, and purified RNA molecules are extracted from these particles.  Once introduced into human cells, the purified RNA molecules induce viral protein synthesis and viral genome replication.  Which of the following is the most likely cause of this patient’s symptoms?

A 9-year-old girl is brought to the emergency department following a 3-minute generalized seizure.  She has had a fever with a worsening headache for 3 days.  The girl has no medical problems and is not taking any medications.  She lives with her parents and pet cat and has traveled to Canada and England.  On physical examination, she is disoriented and has difficulty staying awake.  Her speech is difficult to understand due to inappropriate word choice.  She is admitted to the intensive care unit for further workup.  The patient becomes unresponsive the following day and dies overnight.  Autopsy reveals bilateral, hemorrhagic necrosis of the inferior and medial temporal lobes.  This patient’s condition was most likely caused by infection with which of the following?

A 54-year-old man comes to the emergency department due to worsening fever, cough, and shortness of breath.  The patient initially presented a week ago with a fever, headache, myalgia, nasal discharge, and a sore throat.  He tested positive for influenza A and was discharged home with symptomatic treatment.  The patient’s symptoms gradually improved, but over the past 2 days he has had high fevers, cough productive of yellowish sputum, sharp right-sided chest pain, and shortness of breath.  The patient has no other medical problems and does not use tobacco, alcohol, or illicit drugs.  Temperature is 38.9 C (102 F), blood pressure is 118/66 mm Hg, and pulse is 110/min.  Physical examination reveals right-sided lung crackles.  Chest x-ray reveals infiltrates in the right lung.  Which of the following pathogens is most likely to be isolated from this patient’s sputum?

A 6-year-old boy is brought to the emergency department by his parents with a 2-day history of fever and headaches.  The parents report that he vomited once this afternoon.  All of his vaccinations are up-to-date and he has no significant past medical history.  His temperature is 38.7 C (102 F).  Examination shows mild pharyngeal erythema in addition to neck stiffness.  Cerebrospinal fluid analysis reveals the following:

Which of the following infectious agents is most likely to have caused this patient’s illness?

A 63-year-old woman comes to the office due to shortness of breath and cough productive of thick, green sputum for the past 2 days.  The patient has chronic obstructive pulmonary disease and mild dyspnea on exertion at baseline.  The dyspnea has worsened over the past several days, and she is now also short of breath at rest.  Other medical conditions include hypertension, gastroesophageal reflux, and seasonal allergies.  She has been using her albuterol rescue inhaler more frequently than usual.  The patient says she has cut down to 4 or 5 cigarettes per day and is trying to quit.  Pulse oximetry shows 91% on room air.  She appears to be in mild respiratory distress.  Lung examination demonstrates diffuse bilateral wheezes.  Chest x-ray reveals hyperinflation and increased bronchial markings.  There is no evidence of alveolar infiltrates in either lung.  Which of the following is most likely responsible for the acute worsening of this patient’s symptoms?

An 18-month-old boy is brought to the physician by his parents for fever, runny nose, and sore throat.  The physician reassures the parents and recommends supportive care with plenty of fluids.  He sends them home with instructions to follow up if the boy’s symptoms worsen.  Two days later, the infant is brought to the emergency department with persistent fever, brassy cough, and difficulty breathing.  Physical examination reveals stridor.  Which of the following pathogens is most likely responsible for this patient’s condition?

An outbreak of infection with a seasonal influenza A strain is reported in a small community.  The strain is similar to the one that circulated in the preceding year.  Approximately 70% of the adult population in that community remains uninfected, despite prolonged and repeated exposure to ill contacts.  Which of the following is the most important host factor in preventing influenza infection in these subjects?

A 53-year-old man comes to the emergency department due to progressively worsening shortness of breath, nonproductive cough, and low-grade fevers over the past 2 weeks.  He has not had a runny nose or sore throat and does not recall any sick contacts.  He received a lung transplant for idiopathic pulmonary fibrosis 4 months ago.  His medications include immunosuppressants and trimethoprim-sulfamethoxazole.  Temperature is 37.8 C (100 F).  Chest x-ray reveals diffuse interstitial infiltrates bilaterally.  A decrease in pulmonary function is noted on testing.  A lung biopsy specimen is shown below.

Which of the following best characterizes the organism most likely responsible for this patient’s current condition?

A 22-year-old woman comes to the emergency department with dysuria, vulvar pain, and pruritus.  The patient has also felt feverish and fatigued for the last few days.  She has a new sexual partner and has been using a hormonal vaginal ring for contraception.  Physical examination reveals bilateral inguinal lymphadenopathy and tender vesicular lesions covering the labia majora and perineum.  Which of the following conditions will most likely result as a sequela of this patient’s infection?

A 32-year-old man is brought to the emergency department due to fever, agitation, and disorientation.  According to his family, the patient has been acting strangely and seeing and hearing things that do not exist.  He also drools excessively and has severe, painful throat spasms when he attempts to eat or drink.  The patient returned 4 weeks ago from a camping trip in New Mexico.  Physical examination shows a confused and combative patient with mydriasis and neck rigidity.  Which of the following acts as an entry receptor for the causative agent of this patient’s condition?

A 61-year-old man comes to the office due to a painful rash on his face for the past 3 days.  The patient has no other medical conditions and takes no medications.  He is a retired schoolteacher and lives with his wife.  Skin examination findings are shown in the exhibit.  The remainder of the physical examination shows no abnormalities.  Which of the following events most likely immediately preceded the appearance of skin rash in this patient?

A 23-year-old man with no prior history of seizure is brought to the emergency department after experiencing a generalized tonic-clonic seizure.  His roommate says that the patient has had a fever and headache for the past 2 days and that today he was talking nonsensically.  The patient works for a fast-food chain and occasionally smokes cigarettes on his breaks.  He does not use alcohol or illicit drugs.  Temperature is 38.6 C (101.5 F).  MRI of the brain shows swelling of the temporal lobes.  CSF analysis is most likely to reveal which of the following patterns?

 16-year-old previously healthy boy is brought to the office due to fever, malaise, and sore throat.  The patient says that he gets tired even by simply getting out of bed.  Physical examination shows palatal petechiae, cervical lymphadenopathy, and splenomegaly.  His peripheral blood smear is shown in the image below.

Horse erythrocytes agglutinate when exposed to the patient’s serum.  The agent causing this patient’s disease is most strongly associated with which of the following malignancies?

A 35-year-old man comes to the office due to a rash and weight loss.  He uses intravenous heroin.  Examination shows tender anterior and posterior cervical lymphadenopathy.  White plaques are present over the buccal mucosa and soft palate.  Skin examination demonstrates a widespread rash consisting of lesions similar to that shown in the image below.

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