**Core Concept**
Nephrotic syndrome is characterized by massive proteinuria, hypoalbuminemia, and edema. The underlying mechanism of edema in nephrotic syndrome is related to the loss of serum albumin, which is the primary osmotic pressure in the blood vessels.

**Why the Correct Answer is Right**
Edema in nephrotic syndrome is mainly due to the loss of serum albumin, which normally helps to maintain the osmotic pressure in the blood vessels. When albumin is lost in the urine, the osmotic pressure is decreased, allowing fluid to leak out of the blood vessels into the interstitial space, resulting in edema. This is known as the "oncotic pressure" mechanism.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because while sodium and water retention may contribute to edema in nephrotic syndrome, it is not the primary mechanism.
* **Option B:** This option is incorrect because hyperlipidemia is a feature of nephrotic syndrome, but it is not the main cause of edema.
* **Option C:** This option is incorrect because while renal failure can lead to edema, it is not the primary mechanism in nephrotic syndrome.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the loss of serum albumin in nephrotic syndrome leads to decreased oncotic pressure, which is the primary mechanism of edema in this condition. This concept is crucial for understanding the pathophysiology of nephrotic syndrome.

**Correct Answer:** C. Loss of serum albumin.