Which of the following is a common narcotic?

Heroin is a well-known narcotic that is classified as an opioid. It is derived from morphine, which is obtained from the opium poppy. The use of heroin is associated with its potent analgesic and euphoric effects, which lead to its high potential for addiction and abuse. Heroin functions by binding to the body's opioid receptors, resulting in pain relief and a sense of euphoria, but also carries significant risks such as overdose and various health complications.

In the context of other substances listed, they do not fall under the category of narcotics in the same way heroin does. PCP (phencyclidine) is primarily known as a dissociative anesthetic, Ketamine has similar dissociative properties but is often used in clinical settings for anesthesia and pain relief, and MDMA (3,4-methylenedioxy-methamphetamine), commonly known as ecstasy, is a psychoactive drug that primarily affects serotonin levels and is associated with mood enhancement rather than the narcotic effects seen with heroin. Understanding these classifications helps in recognizing the specific dangers and therapeutic uses associated with different drugs.