When a lumbar puncture is performed to sample cerebrospinal fluid, which of the following external landmarks is the most reliable to determine the position of the L4 vertebral spine?

The iliac crests are used as a landmark for locating the position of L4 to L5 for a lumbar puncture; they are identified and traced medially toward the vertebral column. The inferior angles of the scapulae lie at vertebral level T7; the lowest ribs lead one to T12; a line between the posterior superior iliac spines crosses vertebral level S2; the posterior inferior iliac spines lie below S2.