What is Prolapsed intervertebral Disc (PID) is most common at - and explain in detail?
L5 - SI disc space is the most commonly involved space in disc prolapse.

"Lumbar intervertebral disc herniation is most common at L5 - SI and slightly less common at L4 - L5, and rare at L3 - L4 or above".
"The disc at L5 - Si is most commonly involved, followed in order by L4 - L5 and L3 - L4". — Pocket book of orthopaedics