"Jack: "My brother was sick and went to the d..." joke

Jack: "My brother was sick and went to the doctor."

John: "Is he feeling better now?"

Jack: "No, he has a broken arm."

John: "How did he break it?"
Jack: "Well, the doctor gave him a prescription and told him no matter what happened, to follow that prescription. And the prescription blew out of the window."

John: "How did he break his arm?"

Jack: "He fell out of the window trying to follow the prescription."