ARNOLD KLEIN EXPOSES A MIJAC SECRET: Singer's doc claims he liked to urinate in front of people as a joke.

       TMZ.com's Harvey Levin reportedly spent three hours last Sunday with Dr. Klein, Jackson's best friend and doctor for decades.  Dr. Klein said Jackson got a kick out of tinkling in front of people.      

       "Klein says there were numerous times when Jackson would pee in the doctor's home -- in a cup!!! -- around groups of people, including children," TMZ reported. "Klein says Jackson would also pee in front of other people in his office."       

       According to files from the 1993 Santa Barbara molestation investigation, the young accuser was able to describe -- in explicit detail -- the discoloration on Jackson's genital area ... the result of Vitiligo, a skin discoloration disorder.      

       Dr. Klein says the boy saw Jackson peeing and that's why he could identify the genital markings.