The spinal cord usually ends at the inferior border of L1 or the superior border of L2. Therefore, inserting the needle between L3 and L4 or L4 and L5 is relatively safe. Using any level above L3 for a lumbar puncture would greatly increase the risk of damaging the spinal cord during the spinal tap. This level corresponds to the lumbar cistern, where the cauda equina resides. The pressure from the inserted needle pushes the cauda equina out of harm's way.

The line connecting the top of the two iliac crests. the supracristal line, passes through the spinous process of the L4 vertebra.

Well done! Continue to question 2.