Michael Jackson's Skin Doc Arnie Klein -- The Medical Board Is On My Ass

It ain't lookin' good for Dr. Arnie Klein ... Michael Jackson's dermatologist is under investigation by the Medical Board of California -- reportedly for pumping the singer full of prescription painkillers -- and now his lawyer has officially jumped ship.

Klein just posted a letter on his Facebook page, apparently sent from his former attorney -- advising Klein that he's been subpoenaed by the Board and has to appear for questioning on Dec. 15.

The letter does not explain the nature of the probe -- but according to TheDailyBeast.com, officials want to know more about allegations that Klein over-prescribed Demerol to MJ.

The Board reportedly wants to question Klein about the possibility that he self-prescribed narcotics, used false names on narcotics prescriptions, illegally handed out samples of dangerous drugs to his famous patients, and may have committed gross negligence by practicing while afflicted with multiple sclerosis.

Klein has always strongly denied any wrongdoing ... and thus far, has not been accused of any criminal conduct in the death of Michael Jackson.

As for Klein's lawyer, Herbert L. Weinberg -- dude doesn't explain why he's bailing on Arnie ... but recommends a couple of "fine lawyers" who could take over.