**Core Concept:** The Donnan effect is a phenomenon that describes the movement of ions across the semipermeable capillary wall, causing osmotic pressure changes and fluid shifts. This occurs due to the presence of charged proteins and other molecules in the extracellular fluid, which affect the distribution of ions.

**Why the Correct Answer is Right:** The correct answer is **D** because the Donnan effect is primarily driven by the presence of charged proteins and other molecules in the extracellular fluid, which leads to an imbalance of ions across the capillary wall. This results in osmotic pressure changes and fluid shifts, altering the distribution of ions and causing the phenomenon we call the Donnan effect.

**Why Each Wrong Option is Incorrect:**

A. This option is incorrect because the Donnan effect is not solely due to the movement of water across the capillary wall. It involves the movement of ions as well, driven by the imbalance caused by charged proteins and molecules in the extracellular fluid.

B. This option is incorrect because the Donnan effect is not solely due to the movement of ions across the capillary wall. It is a result of the imbalance caused by charged proteins and molecules in the extracellular fluid.

C. This option is incorrect because the Donnan effect is not solely due to the movement of charged ions across the capillary wall. It is a result of the imbalance caused by charged proteins and molecules in the extracellular fluid.

**Clinical Pearl:** Understanding the Donnan effect is crucial in understanding fluid and electrolyte balance in the body, particularly in conditions like edema and shock where alterations in capillary permeability can significantly impact tissue health.

**Correct Answer:** **D** The Donnan effect is a result of the imbalance of ions across the capillary wall due to the presence of charged proteins and molecules in the extracellular fluid. This phenomenon plays a significant role in maintaining fluid and electrolyte balance in the body.