**Core Concept**
The question is testing the student's knowledge of cerebrospinal fluid (CSF) composition and its differences from plasma. CSF is a clear, colorless fluid that surrounds the brain and spinal cord, and its composition is unique compared to plasma.

**Why the Correct Answer is Right**
The correct answer is related to the fact that CSF has a lower concentration of certain substances compared to plasma. This is due to the blood-brain barrier (BBB), which restricts the passage of certain molecules from the bloodstream into the CSF. The BBB is a highly selective permeability barrier that protects the brain from harmful substances.

**Why Each Wrong Option is Incorrect**
**Option A:** Glucose - In CSF, glucose levels are typically 60-70% of those in plasma, due to the limited glucose transport across the BBB.
**Option B:** Proteins - CSF has a lower concentration of proteins compared to plasma, mainly due to the restricted transport of large molecules across the BBB.
**Option C:** Sodium - Sodium levels in CSF are lower than in plasma, due to the active transport of sodium ions out of the CSF.

**Clinical Pearl / High-Yield Fact**
A classic exam trap is to remember that CSF glucose levels are approximately 60-70% of plasma glucose levels, and that CSF protein levels are typically 1/10th of plasma protein levels.

**Correct Answer:** D.