A 61-year-old man has 4 days of high fever, productive cough, and profuse watery diarrhea. He appears confused, sodium is 121 mEq/L, creatinine 1.8 mg/dL, and chest radiograph shows dense right lower lobe consolidation. He returned from a conference held in a hotel with a cooling tower last week. Most likely pathogen?

• A Streptococcus pneumoniae
• B Mycoplasma pneumoniae
• C Chlamydophila psittaci
• D Legionella pneumophila ✓

Explanation

Legionnaires disease combines severe lobar pneumonia with prominent extrapulmonary features: gastrointestinal upset with diarrhea, hyponatremia from inappropriate ADH secretion, renal impairment, and encephalopathy. Transmission is via aerosols from contaminated water systems including cooling towers and hotel plumbing, matching the travel history. Mycoplasma causes insidious tracheobronchitis in younger adults without hyponatremia or diarrhea, pneumococcus lacks this extrapulmonary constellation, and psittacosis follows bird contact.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.