A Perfectly Good Excuse

A cop saw a car driving erratically and pulled the driver over. "Sir, I need you to breath in this breathalizer for me," said the policeman.
The guy said, "I can't do that, office. I'm an asthmatic. If I do that, then I'll have a really big asthma attack." 
The cop said, "Okay, then I need you to come down to the station with me and we'll do some blood work." 
The guy said, "I can't do that either. I'm a hemophiliac. If I do that, then I will bleed to death." 
The cop said, "Fine. Then I need a urine sample from you." 
The guy replied, "I can't do that either. I'm diabetic. If I do that then my sugar will get REALLY LOW and I may die." 
The cop then said, "Okay, okay! Then I need you to step out of the car and walk this white line." 
The guy said, "Sorry, but I can't do that either." 
The frustrated cop said, "Why not!?" 
The guy said, "Because I'm drunk."