I told my doctor I had a severe case of amnesia.

This joke tickles our funny bone by playing with the inherent absurdity of a doctor asking a standard diagnostic question to someone who, by definition, cannot answer it. The humor mechanism here is a blend of irony and situational comedy. The doctor's question, "Have you had this before?", is a perfectly sensible inquiry in almost any other medical scenario, designed to gauge the history and recurrence of a condition. But when faced with a patient suffering from severe amnesia, a condition characterized by significant memory loss, the question becomes hilariously futile.

The punchline, "Had what?", brilliantly underscores the patient's predicament and the doctor's oversight, highlighting the complete lack of recall that defines amnesia. This condition, often depicted dramatically in movies, is a real neurological or psychological disorder affecting memory. Doctors truly do rely on a patient's history to understand ailments, making the setup of this joke a clever subversion of a very common and practical medical interaction. It's a delightful example of how a simple premise can lead to a big laugh by twisting expectations.