**Question:** Colloidal osmotic pressure of plasma is by

A. Albumin
B. Globulin
C. Fibrinogen
D. Prothrombin

**Core Concept:**
The colloidal osmotic pressure (COP) is a measure of the osmotic pressure exerted by plasma proteins, which helps maintain the balance of fluids within body tissues and organs. Colloidal osmotic pressure is influenced by proteins like albumin, globulin, and fibrinogen, which are essential components of the plasma.

**Why the Correct Answer is Right:**
The correct answer, **A. Albumin**, is responsible for the majority of the colloidal osmotic pressure in plasma. Albumin is a globular protein with a high affinity for binding to various molecules, including ions, hormones, and drugs. This binding capacity contributes to the maintenance of plasma osmotic pressure and helps prevent the leakage of fluids from blood vessels into tissues, leading to edema.

**Why Each Wrong Option is Incorrect:**
Option B, **Globulin**, is an incorrect choice because it is a group of plasma proteins that includes immunoglobulins, alpha-1-antitrypsin, and other proteins. While globulins contribute to the total protein content of plasma, they do not account for a significant portion of the colloidal osmotic pressure.

Option C, **Fibrinogen**, is also incorrect because it is a precursor protein for fibrin formation, which is involved in blood clotting, not osmotic pressure. Fibrinogen does not contribute significantly to plasma osmotic pressure.

Option D, **Prothrombin**, is an incorrect choice because it is a plasma protein involved in the coagulation cascade, not osmotic pressure. Prothrombin does not contribute to maintaining plasma osmotic pressure.

**Clinical Pearl:**
Understanding the role of plasma proteins in maintaining plasma osmotic pressure, particularly albumin, is essential for clinical practice. A decrease in albumin levels, such as in malnutrition or liver disease, can lead to decreased plasma osmotic pressure and increased risk of fluid leakage from blood vessels into tissues, causing edema.