Review Question - QID 215131

A 65-year-old man presents to the urgent care clinic with 3 days of fever and a productive cough. He has had 4 prior hospitalizations for pneumonia in the past 6 months. He also reports lower back pain that is worse at night over the past 2 months. He denies diarrhea or skin rash. His medical history includes hypertension and hyperlipidemia. His medications are aspirin, amlodipine, and atorvastatin. He is a former smoker with a 5 pack-year smoking history and denies illicit drug use. He is in a monogamous sexual relationship with his wife of 40 years. The patient’s temperature is 103.2°F (39.6°C), blood pressure is 114/72 mmHg, pulse is 100/min, and respirations are 20/min. Physical exam reveals a thin-appearing man with pale conjunctivae. There are late inspiratory crackles and whispered pectoriloquy over the right lower lung field. Laboratory results are shown below:

Hemoglobin: 9.5 g/dL
Hematocrit: 28.6%
Na+: 138 mEq/L
Cl-: 98 mEq/L
K+: 4.8 mEq/L
HCO3-: 18 mEq/L
BUN: 32 mg/dL
Glucose: 140 mg/dL
Creatinine: 2.4 mg/dL
Ca2+: 12.3 mg/dL

What is the most likely underlying etiology of this patient’s presenting symptoms?