**Core Concept**
Nephrotic syndrome is characterized by massive proteinuria, hypoalbuminemia, and edema. The edema in nephrotic syndrome is primarily due to a decrease in oncotic pressure caused by the loss of serum albumin, which is a major contributor to the blood's oncotic pressure.

**Why the Correct Answer is Right**
The correct answer is related to the pathophysiology of edema in nephrotic syndrome. Albumin is a large protein molecule that has a high oncotic pressure. When albumin is lost in the urine due to massive proteinuria, the oncotic pressure of the blood decreases, leading to fluid leakage from the blood vessels into the interstitial space, resulting in edema. This mechanism is often referred to as the "loss of oncotic pressure" hypothesis.

**Why Each Wrong Option is Incorrect**
**Option A:**
This option is incorrect because hyperlipidemia is a secondary effect of nephrotic syndrome, not the primary cause of edema.

**Option B:**
This option is incorrect because increased sodium reabsorption is a compensatory mechanism to counteract the loss of oncotic pressure, but it is not the primary cause of edema.

**Option C:**
This option is incorrect because while renal vein thrombosis can cause nephrotic syndrome, it is not the primary cause of edema in this condition.

**Clinical Pearl / High-Yield Fact**
In nephrotic syndrome, the edema is often described as pitting edema, and it typically involves the face, hands, and feet. The edema is due to the loss of serum albumin, which decreases the oncotic pressure of the blood.

**Correct Answer:** C. Decreased oncotic pressure due to loss of serum albumin.