**Question:** Major contributor to plasma osmolarity is:

A. Glucose
B. Urea
C. Electrolytes
D. Albumin

**Core Concept:**
Plasma osmolarity refers to the total concentration of solutes in plasma, which influences osmotic pressure and fluid distribution within the body. Osmolarity is primarily determined by the concentration of solutes like glucose, urea, electrolytes, and proteins like albumin.

**Why the Correct Answer is Right:**
The correct answer is D. Albumin, as it is the most abundant protein in plasma and contributes significantly to total plasma solute concentration, thus affecting osmolarity. Albumin is involved in maintaining colloid osmotic pressure and helps prevent fluid shifting from the intravascular space to the interstitial and extravascular spaces.

**Why Each Wrong Option is Incorrect:**
A. Glucose (Option A) is also a significant contributor to plasma osmolarity, but albumin has a greater impact due to its higher concentration and role in osmotic pressure.
B. Urea (Option B) is another osmotic determinant but has a lesser impact compared to albumin, as urea is produced by the liver and cleared by the kidneys, maintaining a relatively stable concentration.
C. Electrolytes (Option C) are also involved in osmolarity, but their contribution is less significant compared to albumin due to their lower concentration and lesser impact on osmotic pressure.

**Clinical Pearl:**
Understanding the importance of albumin in plasma osmolarity is crucial for clinical practice, particularly in managing patients with altered fluid balance, osmotic diuresis, or conditions affecting albumin synthesis, transport, or clearance, such as liver disease or malnutrition.

**Correct Answer:** D. Albumin