This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Spinal, Epidural, & Caudal Blocks, Subject name: Anaesthesia
While performing a lumbar puncture, a snap is felt just before entering into the epidural space. This is due to piercing of which structure? 
Select and explain. 
A. Ligamentum flavum, B. Supraspinous ligament, C. Duramater, D. Posterior longitudinal ligament
A: Ligamentum flavum
Answer: A: Ligamentum flavum 
Explanation: Ans. a. Ligamentum flava (Ref: Gray's 40/e p729, Snell's 9/e p705)While performing a lumbar puncture, a snap is felt just before entering into the epidural space. This is due to piercing of ligamentum flava.Lumbar PunctureThe depth to which the needle will have to pass varies from 2.5 cm or less in a child to as much as 10 cm in obese adults.In caudal anesthesia, needle pierces skin & fascia & sacrococcygeal membrane that fills in the sacral hiatus.The membrane is formed of dense fibrous tissue & represents fused supraspinous & interspinous ligaments as well as ligamentum flavum. A distinct feeling of 'give' is felt when the ligament is penetratedQ.During lumbar puncture, the needle passes through the following anatomic structures before it enters the subarachnoid space:SkinQSubcutaneous tissueQSupraspinous ligamentQInterspinous ligamentQLigamentum flavumQDura materQArachnoid materQ