A 68-year-old man presents to the emergency department with a 3-week history of fatigue, night sweats, and intermittent fevers. He reports feeling progressively weak and has developed shortness of breath with mild exertion. His past medical history is significant for rheumatic heart disease, for which he underwent a mechanical mitral valve replacement 8 years ago. He is compliant with warfarin for anticoagulation. He also has hypertension, which is well-controlled. Four weeks ago, he had a dental cleaning and a molar extraction, but he does not recall taking any prophylactic antibiotics. He denies chest pain, drug use, and recent travel.

Initial laboratory studies are as follows:

WBC: 13,500/mm³ (4500-10,000)

Segmented neutrophils: 80% (54-62)

Hemoglobin: 10.8 g/dL (13.5-17.5)

Hematocrit: 33% (41-53)

Platelets: 130,000/mm³ (150,000-400,000)

INR: 2.7 (therapeutic for mechanical valve)

Na⁺: 138 mEq/L (136-146)

K⁺: 4.1 mEq/L (3.5-5.0)

BUN: 22 mg/dL (7-18)

Creatinine: 1.3 mg/dL (0.6-1.2)

ESR: 95 mm/hr (0-22)

CRP: 12.5 mg/dL (<1.0)