Lumbar puncture for adults is done at:

Appeared in: Combined AIIMS - 2020 (Jodhpur, Bhopal, Patna, Raipur)

Explanation

In adults, the spinal cord typically ends at the L1-L2 vertebral level, an area known as the conus medullaris.
To prevent injury to the spinal cord, a lumbar puncture must be performed in the subarachnoid space below this level.
The L4-L5 interspace is a wide, easily palpable space located safely below the end of the spinal cord, making it an ideal site for the procedure.
A line drawn between the iliac crests (Tuffier's line) serves as a reliable surface landmark for locating the L4-L5 interspace.

Option A: This level is where the spinal cord (conus medullaris) typically terminates in an adult. Inserting a needle here carries a very high risk of causing direct, permanent damage to the spinal cord.
Option C: While the L5-S1 space is below the spinal cord, it is often narrower and more difficult to access than the L3-L4 or L4-L5 spaces. It is not a preferred or common site for lumbar puncture in adults.
Option D: This option is incorrect because performing a lumbar puncture at the L1-L2 level is contraindicated and dangerous.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Anatomical landmark for lumbar puncture in adults as background academic context rather than a clinical decision trigger.
Correct site selection is a critical patient safety measure to prevent devastating neurological complications, including paralysis, from a lumbar puncture.
Nurses play a key role in positioning the patient correctly (lateral decubitus or sitting and flexed) to maximize the space between the spinous processes, facilitating a successful and safe procedure.
What if the patient is a neonate? In newborns and infants, the spinal cord ends lower, around the L3 vertebra. Therefore, the puncture site is chosen even lower, typically at L4-L5 or L5-S1, to maintain a safe margin.

How to Approach the Question

First, recall the key anatomical fact for this procedure: the level at which the spinal cord terminates in an adult.
The spinal cord ends at L1-L2. This is the 'danger zone' to be avoided.
Evaluate the options based on this fact. Any option at or above L2 is incorrect and unsafe.
Option A (L1-L2) is immediately eliminated as it is in the danger zone.
This also eliminates Option D ('All the above').
Compare the remaining safe options (L4-L5 and L5-S1). Recall or deduce that L4-L5 is the most common and easily accessible site, making it the best answer.

Concept Tested & Keywords

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E6 Nelson Textbook of Pediatrics(2024) — Volume 2 p. 1363-1365

E6 Anatomy Grays 43e Vol 2 Part 2 p. 39-41

Practise the full Combined AIIMS - 2020 (Jodhpur, Bhopal, Patna, Raipur)

Attempt every question from this paper in a timed mock, then review the full solution for each one.