**Core Concept**
Nephrotic syndrome is characterized by significant proteinuria (excretion of >3.5g of protein per 1.73m² body surface area per day), hypoalbuminemia, hyperlipidemia, and edema. The underlying pathophysiology involves damage to the glomerular filtration barrier, leading to excessive loss of albumin and other plasma proteins in the urine.

**Why the Correct Answer is Right**
The correct answer involves the laboratory finding of massive proteinuria, which is a hallmark of nephrotic syndrome. This is due to the increased permeability of the glomerular capillaries to proteins, resulting from damage to the glomerular basement membrane and the podocytes. The loss of large amounts of albumin and other proteins in the urine leads to hypoalbuminemia, which in turn causes a decrease in oncotic pressure and an increase in capillary permeability, resulting in edema.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because while hematuria may be present in certain types of glomerulonephritis, it is not a characteristic finding in nephrotic syndrome.
* **Option B:** This option is incorrect because while a low serum creatinine level may be seen in early stages of nephrotic syndrome, it is not a diagnostic finding of the syndrome.
* **Option C:** This option is incorrect because while hyperkalemia may be seen in certain types of kidney disease, it is not a characteristic finding in nephrotic syndrome.

**Clinical Pearl / High-Yield Fact**
A key clinical pearl to remember is that nephrotic syndrome is characterized by the triad of massive proteinuria, hypoalbuminemia, and hyperlipidemia. This triad is a result of the loss of albumin and other plasma proteins in the urine, leading to a decrease in oncotic pressure and an increase in capillary permeability.

**Correct Answer:** C. Massive proteinuria.