**Core Concept**
The cerebrospinal fluid (CSF) glucose level is a critical parameter in the diagnosis of various neurological conditions. The CSF glucose level is influenced by the glucose gradient between the blood and the CSF.

**Why the Correct Answer is Right**
The CSF glucose level is approximately 60-70% of the plasma glucose level, but more specifically, it is around 2/3 of the plasma glucose level. This is because glucose is actively transported across the blood-CSF barrier by the enzyme glucose transporter 1 (GLUT1), which has a high affinity for glucose. The rate of glucose transport is proportional to the glucose gradient between the blood and the CSF, resulting in a CSF glucose level that is roughly 2/3 of the plasma glucose level.

**Why Each Wrong Option is Incorrect**
**Option A:** Half the plasma glucose - This is an oversimplification, as the actual CSF glucose level is closer to 2/3 of the plasma glucose level.
**Option C:** 1/3 plasma glucose - This is too low and does not accurately reflect the glucose gradient across the blood-CSF barrier.
**Option D:** Same as plasma glucose - This is incorrect because the CSF glucose level is influenced by the glucose gradient and active transport across the blood-CSF barrier.

**Clinical Pearl / High-Yield Fact**
A low CSF glucose level (< 40-50% of plasma glucose) can indicate bacterial meningitis, as bacteria consume glucose and reduce its level in the CSF.

**✓ Correct Answer: B. 2/3 plasma glucose**