A 23-year-old woman comes to the emergency department due to 3 days of fever, chills, cough productive of clear sputum, and pain in her right lower chest with deep inspiration and cough. She has had no sore throat, nasal discharge, or sick contacts. The patient has a history of HIV and takes combination antiretroviral therapy as prescribed. CD4 count was 300/mm³ at routine office follow-up a month ago. She does not use tobacco, alcohol, or illicit drugs. Temperature is 39 C (102.2 F), blood pressure is 115/76 mm Hg, pulse is 104/min, and respirations are 22/min. Oropharyngeal mucosa is normal and there is no cervical lymphadenopathy. Cardiopulmonary auscultation reveals a soft midsystolic murmur at the upper left sternal border and crackles in the right lower posterior chest. Chest x-ray is shown below. Which of the following is the most likely organism responsible for this patient's current symptoms?

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11ec25c8_d443_9b73_bc81_c14488a33f7b_MD0003_00 Patients with HIV are at increased risk for community-acquired pneumonia (CAP), particularly in the setting of advanced disease (CD4 count <200/mm³). Streptococcus pneumoniae causes the majority of cases, likely due to increased rates of colonization and impaired immunity against encapsulated bacteria. Symptoms of bacterial CAP are similar in HIV-infected and uninfected individuals and include fever, pleuritic pain, dyspnea, and productive cough. "Rusty sputum" is classic for pneumococcal pneumonia but may not always be present. Diagnosis is confirmed by finding a lobar, interstitial, or cavitary infiltrate on chest imaging (eg, chest x-ray). Pneumococcal vaccination is recommended for all patients with HIV to reduce the risk of invasive S pneumoniae disease. (Choice A) Invasive aspergillosis often manifests with the classic triad of fever, pleuritic chest pain, and hemoptysis. Imaging usually reveals focal lesions (nodules with or without cavitation). It is often associated with immunosuppression (eg, HIV with CD4 count <50/mm³). This patient has a lobar infiltrate and CD4 count >50/mm³, making a bacterial or viral cause of CAP more likely. (Choice B) Coccidioidomycosis is a fungal infection endemic to the southwestern United States; it often manifests as CAP with hilar lymphadenopathy. S pneumoniae is much more common. (Choices C and G) Klebsiella and Pseudomonas primarily cause nosocomial pneumonia. Although Klebsiella can cause aspiration pneumonia, this patient has no clear features (eg, impaired consciousness, dysphagia) that put her at risk for aspirating. (Choice D) Patients with HIV are at increased risk of tuberculosis, which usually presents with subacute fever, cough, weight loss, and apical pulmonary disease (not lower-lobe pneumonia). (Choice E) Mycoplasma pneumoniae usually presents over days or weeks (not acutely) with headache, malaise, nonproductive cough, and low-grade fever. (Choice F) Pneumocystis pneumonia (PCP) is seen primarily in patients with CD4 counts <200/mm³ and manifests with indolent symptoms of dyspnea, nonproductive cough, and (usually) bilateral, diffuse interstitial infiltrates. This patient with a recent CD4 count of 300/mm³ who takes her antiretroviral therapy as prescribed is unlikely to have PCP. (Choice H) Patients with HIV have higher rates of Staphylococcus aureus CAP, which usually manifests as rapidly progressive necrotizing pneumonia. However, S pneumoniae is far more common. In addition, although S aureus infective endocarditis can cause septic pulmonary emboli or pneumonia, this patient does not have major risk factors (eg, advanced age, poor dentition, injection drug use) for infective endocarditis; her cardiac murmur timing and location are most consistent with an innocent "flow" murmur due to hyperdynamic circulation from fever. Educational objective: Patients with HIV are at increased risk of community-acquired pneumonia. Streptococcus pneumoniae is the most commonly implicated organism.

A 23-Year-Old Woman Comes to the Emergency Department Due to 3