## Core Concept
The question tests the understanding of the pathophysiological mechanisms leading to edema, specifically focusing on the role of plasma osmotic pressure. Plasma osmotic pressure, primarily maintained by albumin, helps retain fluid within the blood vessels. A decrease in this pressure can lead to fluid leakage into the interstitial space, causing edema.

## Why the Correct Answer is Right
The correct answer, **C. Nephrotic syndrome**, is associated with significant proteinuria, leading to a decrease in plasma albumin levels. Albumin is crucial for maintaining the oncotic pressure within blood vessels. When its levels drop, fluid moves from the vascular space into the interstitial space, resulting in edema. This condition exemplifies how a decrease in plasma osmotic pressure can cause edema.

## Why Each Wrong Option is Incorrect
- **Option A:** Liver cirrhosis primarily causes edema through portal hypertension and the resultant fluid shift into the peritoneal cavity (ascites), rather than a direct decrease in plasma osmotic pressure. While cirrhosis can affect albumin production, the primary mechanism of edema in cirrhosis is not a decrease in plasma osmotic pressure.
- **Option B:** Heart failure leads to edema primarily through increased hydrostatic pressure due to the heart's inability to pump blood efficiently. This mechanism is different from a decrease in plasma osmotic pressure.
- **Option D:** Lymphatic obstruction causes edema by impairing the lymphatic system's ability to drain interstitial fluid back into the bloodstream. This mechanism is distinct from a decrease in plasma osmotic pressure.

## Clinical Pearl / High-Yield Fact
A key point to remember is that nephrotic syndrome is a classic cause of edema due to hypoalbuminemia (low albumin levels), which directly decreases plasma osmotic pressure. This condition highlights the importance of albumin in maintaining vascular oncotic pressure and preventing edema.

## Correct Answer: C. Nephrotic syndrome.