http://en.wikipedia.org/wiki/Rectal_prolapse
"Because most sufferers are elderly, the condition is generally under-reported. A common nickname for this condition is "Pink Sock" a term popularized by comedian James Norton, as named after what it resembles.
Porn Star Ava Devine has experienced Rectal Prolapse during filming of a couple of anal sex scenes.
This condition which has occurred in various anal sex scenes is also called the anal rose, or ass tulip."