**Core Concept**
The osmolality of plasma is a measure of the concentration of osmotically active particles per unit of solvent, typically expressed in milliosmoles per kilogram of solvent (mOsm/kg). The main contributors to plasma osmolality are electrolytes, primarily sodium.

**Why the Correct Answer is Right**
Sodium is the primary contributor to plasma osmolality because it is the most abundant electrolyte in the body, with a concentration of approximately 140 mEq/L. Sodium's contribution to osmolality is significant due to its high concentration and its ability to dissociate into two osmotically active particles (sodium ions and chloride ions). This results in a substantial contribution to the overall osmolality of plasma. The other options, glucose, urea, and uric acid, also contribute to plasma osmolality, but to a much lesser extent.

**Why Each Wrong Option is Incorrect**
**Option A:** Glucose contributes to plasma osmolality, but its concentration is typically much lower than sodium's, making it a relatively minor contributor.
**Option C:** Urea is another osmotically active substance in plasma, but its concentration is typically lower than sodium's, making it a less significant contributor to plasma osmolality.
**Option D:** Uric acid is an osmotically active substance in plasma, but its concentration is typically very low, making it a negligible contributor to plasma osmolality.

**Clinical Pearl / High-Yield Fact**
In clinical practice, the osmolality of plasma can be affected by various conditions, such as hypernatremia (elevated sodium levels) or hyponatremia (low sodium levels). It's essential to consider these conditions when interpreting plasma osmolality results, especially in patients with altered mental status or seizures.

**✓ Correct Answer: B. Sodium. Sodium is the primary contributor to plasma osmolality.**