**Core Concept**
Nephrotic syndrome is characterized by a complex pathophysiological process involving renal dysfunction, leading to massive proteinuria, hypoalbuminemia, and edema. The underlying mechanism of edema in nephrotic syndrome is primarily related to alterations in renal function and the body's response to protein loss.

**Why the Correct Answer is Right**
The correct answer, **B. Sodium and water retention**, is due to the complex interplay between renal function and hormonal regulation. In nephrotic syndrome, the kidneys are unable to reabsorb sufficient amounts of protein, leading to massive proteinuria and subsequent hypoalbuminemia. This decrease in albumin levels results in increased levels of atrial natriuretic peptide (ANP), which in turn leads to sodium and water retention. Moreover, the decreased levels of albumin also lead to decreased oncotic pressure in the blood vessels, causing fluid to shift from the intravascular compartment to the interstitial space, exacerbating edema. Furthermore, the kidneys also respond to the low albumin levels by increasing the production of renin, which ultimately leads to increased levels of aldosterone, promoting sodium retention and further exacerbating edema.

**Why Each Wrong Option is Incorrect**
**Option A:** Reduced plasma protein is a consequence of nephrotic syndrome, but it is not the direct cause of edema. The reduction in plasma protein levels leads to decreased oncotic pressure, but it is the subsequent sodium and water retention that directly contributes to edema.

**Option C:** Increased venous pressure can contribute to edema, but it is not the primary mechanism in nephrotic syndrome. The edema in nephrotic syndrome is primarily due to sodium and water retention, rather than increased venous pressure.

**Option D:** Hyperlipidemia is a common finding in nephrotic syndrome, but it is not directly related to the pathogenesis of edema in this condition. The hyperlipidemia is thought to be secondary to the increased levels of low-density lipoprotein (LDL) receptors in the liver, which is a consequence of the low albumin levels.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the edema in nephrotic syndrome is not just due to hypoalbuminemia, but also due to the complex interplay between renal function and hormonal regulation, leading to sodium and water retention. This understanding is crucial for differentiating nephrotic syndrome from other causes of edema, such as heart failure or liver disease.

**✓ Correct Answer: B. Sodium and water retention**