Two coppers stop a car late at night which is proceeding cautiously along the road, and assume the man has been drinking.
"Would you mind providing a breath specimen, by blowing into this bag?"
"Sorry officer,I'm asthmatic, I can't do that"
"O.k. we'll just take you back to the station!"
At the station the man is unable to provide a urine sample, so the oficer suggests a blood sample.
"Sorry officer, I can't do that, I'm a haemophiliac!"
At this stage the officer is getting a little exasperated, and consults his sergeant (a relic of a bygone era).
"RIGHT!" says the sergeant, drawing a chalk line along the floor of the station. "Just walk along that line, and no excuses"
"Sorry officer, I can't do that, I'm pissed!"