Q. A 31-year-old man comes to the emergency department because of a five-day fever associated with anorexia and fatigue. The patient has no significant past medical history. He was born in the United States and has never traveled abroad. He regularly smokes marijuana and recently started to inject heroin. Vital signs are blood pressure 100/50 mm Hg, heart rate 113/min, respiratory rate 20/min, and temperature 38.9 degrees Celsius. Cardiac auscultation reveals an early diastolic murmur over the aortic area. Which of the following is considered the most common causative organism of this patient's infection?