Oedema is caused by fall in plasma proteins below –

**Core Concept**
Decreased plasma oncotic pressure is a primary cause of edema. Plasma proteins, such as albumin, contribute to oncotic pressure by maintaining fluid within the vascular compartment.

**Why the Correct Answer is Right**
The correct answer is related to the concept of oncotic pressure, which is directly proportional to the concentration of plasma proteins. A decrease in plasma protein concentration leads to a decrease in oncotic pressure, resulting in fluid shift from the vascular compartment to the interstitial space, causing edema. This phenomenon is described by the Starling forces, which dictate the movement of fluid across capillary walls.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it doesn't provide a specific value, making it impossible to determine its accuracy.
* **Option B:** This option is incorrect because it suggests a normal plasma protein concentration, which would not lead to edema.
* **Option C:** This option is incorrect because it implies a value below which edema does not occur, which is not accurate.

**Clinical Pearl / High-Yield Fact**
A decrease in plasma albumin concentration by 1 g/dL can lead to a 6 mmHg decrease in oncotic pressure, resulting in a significant increase in edema.

**Correct Answer: 3 g/dL.**