**Core Concept**
Nephrotic syndrome in elderly patients is a condition characterized by massive proteinuria, hypoalbuminemia, hyperlipidemia, and edema. The pathophysiology involves damage to the glomerular filtration barrier, leading to loss of large molecules such as proteins.

**Why the Correct Answer is Right**
The most common cause of nephrotic syndrome in elderly patients is diabetic nephropathy. This is due to the progressive damage to the glomeruli caused by chronic hyperglycemia, which disrupts the normal glomerular filtration barrier. As a result, albumin and other large molecules are lost in the urine, leading to hypoalbuminemia and hyperlipidemia. Diabetic nephropathy is a well-established complication of diabetes mellitus, and its prevalence increases with age.

**Why Each Wrong Option is Incorrect**
* **Option A:** Focal segmental glomerulosclerosis (FSGS) is a common cause of nephrotic syndrome in younger patients, but it is less common in elderly patients.
* **Option B:** Minimal change disease (MCD) is a common cause of nephrotic syndrome in children, but it is rare in elderly patients.
* **Option C:** Membranous nephropathy is a cause of nephrotic syndrome, but it is less common in elderly patients compared to diabetic nephropathy.

**Clinical Pearl / High-Yield Fact**
Diabetic nephropathy is a leading cause of end-stage renal disease (ESRD) in the elderly population. Early detection and management of diabetes mellitus are crucial to prevent the development of nephrotic syndrome and other complications.

**Correct Answer:** D. Membranous nephropathy