Requiem for a Dream

During the summer in Brighton Beach, Brooklyn, widow Sara Goldfarb constantly watches television, particularly infomercials hosted by Tappy Tibbons. After receiving an unexpected phone call that she has won a spot to participate on a television game show, she becomes obsessed with regaining the youthful appearance she possesses in an old photograph. To reach her goal, she goes to a doctor to discuss weight loss. The doctor gives her a prescription for weight-loss amphetamine pills throughout the day and a sedative at night. As the months go by, Sara's tolerance for the pills adjusts and as a result she is no longer able to feel the same high the pills once gave her. When her invitation does not arrive, she increases her dosage from double to triple and, as a result, begins to suffer from amphetamine psychosis.
Meanwhile, Sara's son Harry is young and has an adventurous life with his girlfriend Marion Silver and best friend Tyrone C. Love. He and his friends are all heroin addicts, and Harry funds his habit through petty theft, including from his own mother who turns a blind eye. Tyrone decides that to support themselves, they should enter the illegal drug trade around Coney Island. With the promised money, each addict hopes to achieve their dreams. At first, the trio's drug dealing business thrives. However, Tyrone is imprisoned after fleeing the scene of a drug-gang assassination and Harry uses most of their earned money to post bail. After the assassination, the three find it much more difficult to find heroin due to supply being restricted by the Florida-based wholesalers. Eventually, Tyrone hears that the wholesaler is making a shipment, but the price is doubled and the minimum amount is high. Harry suggests that Marion ask for the needed cash from her psychiatrist in exchange for sex, which she does begrudgingly, and at a cost to her relationship with Harry. Tyrone and Harry go to meet the wholesaler, but the rest of the local low-level dealers are also there, and tensions are running high. There is pushing and shoving and soon a shot rings out. The wholesaler's bodyguards respond by firing indiscriminately into the crowd before departing. Harry returns empty-handed to Marion, who is so desperate that she has turned the apartment upside down searching for scraps, and they have a screaming argument. He leaves her after giving her the number of a pimp who trades heroin for sex. Harry convinces Tyrone that their best option is to drive all the way to Florida to pick up and put their drug dealing business back on track.
After a series of traumatizing hallucinations, Sara flees her apartment and takes a subway to the casting agency in Manhattan to confirm when she will be on the show with Tappy Tibbons. Once at the agency, the staff try to reassure her while they wait for paramedics. Sara is involuntarily committed to a psychiatric ward where she is given a series of degrading treatments. When none of these work, the doctor persuades a barely lucid Sara to sign approval for electroconvulsive therapy.
On the way to Miami, Harryâs increasingly infected left arm from his heroin injections forces them to visit a hospital. A doctor notices the symptoms of drug abuse and Harry and Tyrone are arrested. Back in New York, Marion meets with the dealer who gives her drugs in exchange for sex and invites her to join a "party" that weekend if she wants more.
In the climax of the film, we see a series of rapidly intercut scenes showing the depths to which our four protagonists have sunk. Tyrone is doing hard labor in jail, being taunted by prison guards, and unable to sleep due to drug withdrawal. Harry is transferred from prison to a hospital and his arm is amputated. Sara undergoes several rounds of electroconvulsive therapy. Marion returns to the dealer's apartment, where the "party" is a private sex show in the middle of a crowded room of braying men, in which Marion has to perform humiliatingly.
Sara's friends come to the hospital to see her but she doesn't react to them, and both can later be seen outside on a bench crying. Harry wakes after the amputation and weeps. Tyrone lies in his prison bed haunted by memories of his mother. Marion lies on her sofa serenely clutching her bag of heroin. That night, Sara has a dream in which she wins the grand prize in game show with Harry as the guest of honor.

Question: Which of Harry's arms is infected?
Answer: Left