What is the missing number?

A pretty young blonde visiting her new doctor for the first time found herself alone in a small waiting room. She began undressing nervously, preparing herself for the upcoming examination. Just as she draped the last of her garments over the back of a chair, a light rap sounded on the door and a young doctor strode in.
Coming to an abrupt halt, the doctor looked his nude patient up and down carefully and with considerable appreciation.
"Miss Smith," he said finally, "it seems quite obvious to me that until today you have never undergone an eye examination."