MLB superstar Ryan Braun -- the 2011 National League MVP -- says he will not hide from allegations he used performance-enhancing drugs ... telling TMZ, "My day will come."

TMZ broke the story ... MLB sources say Braun's dirty test was caused by medication for a private medical issue.

Braun arrived at LAX yesterday -- and when we asked about the situation ... the Milwaukee Brewers star told us, "I can't get into it man ... trust me, I want to ... my day will come."

Ryan Braun's positive test for banned substances was caused by medication he's taking for a private medical issue -- NOT performance enhancing drugs ... this according to sources directly connected with Major League Baseball.

We're told the reigning National League MVP is adamant he has not taken drugs or steroids of any kind.

One source simply told us ... "The medication contained banned substances resulting in the positive test."  We're also told prior to taking the dirty test, Braun had always tested negative for banned substances.

The nature of Braun's medical ailment is unclear.

It's also unclear if the Milwaukee Brewers star knew the substances were in his medication before he took it -- though it seems highly unlikely.