An embarrassing medical condition which you can only bring yourself to reveal to the doctor as you are about to leave his surgery.
A doorknob condition is not, as you might expect, related to the maintenance record of door furniture, but is any of a class of medical conditions. This colloquial term doesn't always apply to a particular condition. Rather, it is a combination of the patient and their complaint which causes it to be a 'doorknob' case.
We've all1 been there; 'slipped' in the shower, or accidentally been bitten on the genitalia by the family pet. You book an appointment with your friendly GP, walk into his office, waffle on about life, the universe, the price of fish and everything else.
When do you finally pluck up the courage to mention your anal seepage, or grazed scrotum?
When you're about the leave the office, and have your hand on the doorknob.
That is why it's a doorknob condition.
1 Haven't we?