A 19-year-old presents at the emergency department with high fever, severe headache, nausea, and stiff neck for 3 days. The attending physician suspects meningitis and obtains a sample of CSF using a lumbar puncture. From which of the following spaces was the CSF collected?

**Core Concept**
The attending physician performed a lumbar puncture to collect cerebrospinal fluid (CSF) for diagnostic purposes. The procedure involves puncturing the meninges, the protective membranes surrounding the central nervous system, to access the subarachnoid space where CSF is produced and circulates.

**Why the Correct Answer is Right**
During a lumbar puncture, the needle is inserted between the L3-L4 or L4-L5 intervertebral spaces, allowing access to the subarachnoid space. This space is located beneath the arachnoid mater, one of the meningeal layers, and contains the CSF produced by the choroid plexus in the ventricles of the brain. The CSF circulates through the subarachnoid space, bathing the brain and spinal cord, before being reabsorbed into the bloodstream.

**Why Each Wrong Option is Incorrect**
**Option A:** The cisterna magna is a large cerebrospinal fluid-filled space located in the posterior cranial fossa, but it is not accessible through a lumbar puncture.
**Option B:** The ventricles are cavities within the brain where CSF is produced, but they are not where CSF is collected during a lumbar puncture.
**Option C:** The subdural space is located between the dura mater and the arachnoid mater, but it is not where CSF is collected during a lumbar puncture.

**Clinical Pearl / High-Yield Fact**
The lumbar puncture site should be carefully selected to avoid puncturing the dura mater, which can lead to a traumatic tap and alter CSF analysis results. The L3-L4 or L4-L5 interspace is usually chosen due to its more horizontal orientation, making it easier to access the subarachnoid space.

**Correct Answer:** C. Subarachnoid space.

✓ Correct Answer: C. Subarachnoid space