Endocarditis Quiz: Ace Your Exams

A 36-year-old man presents to the emergency department with fever, chills, and cough of three days duration. The patient uses intravenous heroin regularly and reports he last used heroin earlier today. He is currently living in a homeless shelter. Temperature of 38.7 °C (101.6 °F), pulse is 105/min, respirations are 22/min, blood pressure is 100/63 mmHg, and oxygen saturation is 91% on room air. On physical examination, the patient appears disheveled. Erythematous lesions are noted on the chest and arms bilaterally. Rales are appreciated in the lungs bilaterally, and a holosystolic murmur is appreciated at the left lower sternal border. Laboratory values are demonstrated below:  
 

Laboratory value	Result
Complete Blood Count, Serum
Hemoglobin	12.5 g/dL
Hematocrit	40.5%
Leukocyte count	18,100 /mm3
Platelet count	250,000/mm3
Inflammatory Markers, Serum
ESR	44 mm/hr
CRP	18 mg/L
Electrolytes, Serum
Sodium	131 mEq/L
Potassium	3.6 mEq/L
Chloride	94 mEq/L
Bicarbonate	20 mEq/L
Calcium	8.4 mg/dL

Blood is drawn and sent for culture, and the patient is started on empiric antibiotics. Three days later, 3 of 4 blood cultures obtained on admission grow Gram-positive, coagulase-positive bacteria. Which of the following is the most common initial site of infection in this patient?