I'd be inclined to invent a 'medial problem' involving 'appointments', 'tests' 'test results' 'therapy' and so on.
Your employer shouldn't be so rude as to ask what the problem is, if he does, say it's 'personal' and walk away, he'll be too embarassed to mention it again!
Just one thing before you use this scheme - make sure your company don;t ask for proof of appointments - letters or appointment cards. If they do, pull the 'personal' angle again, should get you out of it.
Good luck!