**Core Concept**
Plasma osmolarity is the measure of the concentration of osmotically active particles in the blood, which determines the balance of fluids between the intravascular and interstitial compartments. Osmolarity is influenced by the concentration of various solutes, including electrolytes, glucose, and proteins.

**Why the Correct Answer is Right**
Sodium is the primary contributor to plasma osmolarity, accounting for approximately 80-90% of the total osmotic activity. This is because sodium ions are the most abundant electrolyte in the blood, with a concentration of around 135-145 mmol/L. Sodium's significant contribution to osmolarity is due to its high concentration and its ability to freely diffuse across cell membranes, making it a key determinant of the balance of fluids between the intravascular and interstitial spaces.

**Why Each Wrong Option is Incorrect**
**Option A:** Urea is a waste product that contributes to plasma osmolarity, but its contribution is relatively small compared to sodium. Urea's osmotic activity is approximately 5-7 mmol/L, which is significantly lower than sodium's contribution.
**Option B:** Glucose also contributes to plasma osmolarity, but its contribution is typically small and variable, depending on the individual's blood glucose levels. In a normal individual, glucose's osmotic activity is around 5-10 mmol/L, which is much lower than sodium's contribution.
**Option D:** Albumin, a protein, does contribute to plasma osmolarity, but its contribution is relatively small compared to sodium. Albumin's osmotic activity is approximately 20-30 mmol/L, but this is still much lower than sodium's contribution.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the balance of fluids between the intravascular and interstitial compartments is critical for maintaining blood pressure and preventing edema. Understanding the contributions of various solutes to plasma osmolarity, particularly sodium, is crucial for managing patients with fluid imbalances and electrolyte disorders.

✓ Correct Answer: C. Sodium