A doctor observed the following in a young man: aggression, mood swings, depression, reduction of size of the testicles, decreased sperm production, breast enlargement, and premature baldness. He might be abusing which of the following, most probably?

The symptoms described, including aggressiveness, mood swings, testicular shrinkage, decreased sperm production, breast enlargement, and premature baldness, strongly suggest abuse of anabolic steroids. These synthetic variations of testosterone disrupt the body's hormonal balance, leading to a range of physical and psychological effects. While other substances like heroin, marijuana, and morphine can also cause health and behavioral issues, the specific combination of symptoms aligns most closely with the known effects of anabolic steroid abuse.