A 24 year-old male is admitted to the hospital for treatment of endocarditis secondary to intravenous drug use. He was discharged from a methadone clinic 3 weeks ago and he bought some buprenorphine and some other pills off the street. Today his urine drug screen came positive for 6-monoacetylmorphine (6-MAM). Which of the following substances is he most likely abusing at this time?