Dr. Oz's Mystery Bulge -- I Was 'Very Excited' About My Pasta

The Dr. Oz trouser-tent mystery has come to an end ... with the good doc finally admitting why he was sporting some "moose knuckle" in a recent photo shoot.

Oz appeared on "Watch What Happens LIVE" with Andy Cohen last night ... when Andy gave it to him straight -- "Everybody's talking about your bulge."

The bulge in question appeared in the April edition of Good Housekeeping ... while Oz was posing with a bowl of pasta.

"It was very exciting for me to do that," Oz admitted ... saying even his wife was left with a few questions.