**Core Concept**
The glucose concentration in cerebrospinal fluid (CSF) is influenced by the glucose gradient between the plasma and CSF. The rate of glucose transport across the blood-cerebrospinal fluid barrier is a critical determinant of CSF glucose levels.

**Why the Correct Answer is Right**
The correct answer is B. 2/3 of plasma glucose. This is because glucose is transported from the plasma into the CSF by a carrier-mediated mechanism, which is a concentration-dependent process. Under normal conditions, approximately 70-80% of the glucose that enters the CSF is produced by the choroid plexus, while the remaining 20-30% is derived from the plasma. Therefore, CSF glucose levels are typically 2/3 of the plasma glucose concentration.

**Why Each Wrong Option is Incorrect**
* **Option A:** Incorrect because CSF glucose levels are not simply half of the plasma glucose concentration. The glucose gradient between the plasma and CSF is more complex and depends on various factors, including the rate of glucose transport and CSF production.
* **Option C:** Incorrect because CSF glucose levels are not 1/3 of the plasma glucose concentration. This option underestimates the glucose concentration in CSF, which is typically higher than this value.
* **Option D:** Incorrect because CSF glucose levels are not the same as plasma glucose levels. While there is a close relationship between the two, the glucose concentration in CSF is influenced by the glucose gradient and the rate of glucose transport across the blood-cerebrospinal fluid barrier.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that CSF glucose levels can be affected by various factors, including diabetes mellitus, hypoglycemia, and impaired glucose transport across the blood-cerebrospinal fluid barrier. A decrease in CSF glucose levels can be indicative of bacterial meningitis, which can increase glucose consumption and reduce glucose levels in the CSF.

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