A 76-year-old man is brought to the hospital due to a 2-week history of cough, increasing shortness of breath, right-sided chest pain, and fever. The patient also has lost 3 kg (6.6 lbs) during this time. Medical history includes type 2 diabetes mellitus, hypertension, and Alzheimer dementia. Temperature is 39.1 C (102.4 F), blood pressure is 102/68 mm Hg, pulse is 112/min, and respirations are 24/min. Examination shows decreased respiratory excursion, decreased breath sounds, and dullness to percussion on the right side. Laboratory studies reveal leukocytes 22,000/mm³ with 85% neutrophils and hemoglobin 10.5 g/dL. Chest x-ray reveals a moderate pleural effusion. A right lateral decubitus x-ray shows the effusion to be free flowing, and an underlying right lower lobe infiltrate is noted. Thoracentesis yields 750 mL of foul-smelling turbid fluid. Which of the following organisms is most likely responsible for this patient's current condition?

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11ec25c8_d94b_72c0_bc81_fd1d4abc1935_MD0003_00 This elderly patient with Alzheimer dementia likely has aspiration pneumonia complicated by pleural effusion and empyema. The foul-smelling fluid is highly suggestive of infection with anaerobic oral flora, which is a typically polymicrobial infection (eg, Streptococcus intermedius, Fusobacterium spp, Prevotella spp, Bacteroides spp). Empyema (ie, pus in the intrapleural space) represents advanced progression of a complicated parapneumonic effusion. Initial presentation may be similar to uncomplicated pneumonia (eg, fever, cough, shortness of breath, pleuritic chest pain, leukocytosis). However, patients with empyema often have weight loss and a relatively gradual presentation (eg, a week or more); the presentation of anaerobic empyema may be especially prolonged (eg, 2-3 weeks). Accompanying pleural effusion is often suggested by physical examination (eg, decreased breath sounds, dullness to percussion) and confirmed by chest x-ray, where the effusion may appear free flowing (early empyema) or relatively fixed with evidence of loculation (more advanced). Thoracentesis and pleural fluid analysis confirm the diagnosis; turbidity suggests pus (ie, empyema) and putrid or foul-smelling pleural fluid is diagnostic of anaerobic empyema as aerobic organisms do not produce the same pungent, sulfur-containing compounds as anaerobes. (Choice A) Mycobacterium tuberculosis is a relatively rare cause of empyema. It may complicate reactivation of latent tuberculosis and be associated with fevers, cough, and weight loss. However, it is not expected in association with a lower lobe pneumonia (reactivation usually affects the upper lobes) and typically causes symptoms over a prolonged period (sometimes months). In addition, M tuberculosis is highly aerobic and is not expected to cause foul-smelling pleural fluid. (Choices C, D, and E) Streptococcus pneumoniae is among the most common causes of community-acquired empyema, Staphylococcus aureus is a common cause of both community-acquired and hospital-acquired empyema, and Pseudomonas aeruginosa is an occasional cause of hospital-acquired empyema (eg, in patients with cystic fibrosis). Empyema caused by these aerobic organisms does not produce the foul smell associated with anaerobic organisms. In addition, the presentation is typically less gradual (eg, a week rather than 2-3 weeks of symptoms) than empyema caused by anaerobic organisms. Educational objective: Aspiration pneumonia leading to translocation of anaerobic oral flora into the intrapleural space is likely the most common cause of empyema, commonly presenting insidiously (eg, over 2-3 weeks) with weight loss and similar signs and symptoms to uncomplicated pneumonia. Putrid or foul-smelling pleural fluid is diagnostic of anaerobic empyema. References: Parapneumonic pleural effusions and empyema thoracis. (http://www.ncbi.nlm.nih.gov/pubmed/30485002) Anaerobic infections. (http://www.ncbi.nlm.nih.gov/pubmed/29493974)

A 76-Year-Old Man Is Brought to the Hospital Due to a 2-Week