**Core Concept**
Electron microscopy (EM) is a crucial diagnostic tool in renal pathology, allowing for the visualization of ultrastructural details that are not visible under light microscopy. It is particularly useful in distinguishing between various glomerular diseases, such as minimal change disease, membranous nephropathy, and focal segmental glomerulosclerosis.

**Why the Correct Answer is Right**
In renal biopsy studies, EM is essential for identifying characteristic ultrastructural features of specific glomerular diseases. For instance, the presence of subepithelial humps in membranous nephropathy or the effacement of foot processes in minimal change disease can be visualized using EM. This allows for a definitive diagnosis, which is often not possible with light microscopy alone.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because EM is not typically used to diagnose acute tubular necrosis (ATN), which is usually diagnosed based on clinical presentation and light microscopy findings.
* **Option B:** This option is incorrect because EM is not primarily used to diagnose diabetic nephropathy, which is typically diagnosed based on clinical presentation, light microscopy findings, and immunofluorescence.
* **Option C:** This option is incorrect because EM is not typically used to diagnose focal segmental glomerulosclerosis (FSGS), which is usually diagnosed based on clinical presentation, light microscopy findings, and immunofluorescence.

**Clinical Pearl / High-Yield Fact**
When interpreting renal biopsy results, it's essential to remember that EM can provide a definitive diagnosis in cases where light microscopy is inconclusive. This is particularly true for distinguishing between various glomerular diseases.

**Correct Answer:** A. Fibrillary glomerulonephritis. Fibrillary glomerulonephritis is a rare glomerular disease characterized by the presence of fibrils in the glomerular basement membrane, which can only be visualized using electron microscopy.