EXCLUSIVE

Playboy model Katie May died from a simple visit to a chiropractor for an adjustment, which ultimately left her with a fatal tear to an artery in her neck ... according to the L.A. County Coroner.

TMZ obtained a copy of Katie's death certificate, and it clears up a huge mystery surrounding her sudden death in February. The document says she died when a blunt force injury tore her left vertebral artery, and cut off blood flow to her brain.

It also says the injury was sustained during a "neck manipulation by chiropractor." Her death is listed as accidental.

As TMZ first reported, Katie -- known as the "Queen of Snapchat" -- had taken a nasty fall during a photo shoot. She thought she had a pinched nerve and went to an L.A. chiropractor on Friday. She suffered a stroke on Monday, and remained on life support until Thursday.