**Core Concept**
Crescent formation is a hallmark of severe glomerulonephritis, characterized by the accumulation of crescents of proliferating parietal epithelial cells and monocytes/macrophages in Bowman's space, indicating severe glomerular damage and potentially leading to rapid progression of kidney failure.

**Why the Correct Answer is Right**
Crescent formation is most commonly associated with anti-GBM disease (Goodpasture syndrome) and ANCA-associated vasculitis, such as granulomatosis with polyangiitis and microscopic polyangiitis. In these conditions, the immune complex deposition and subsequent activation of the complement system and recruitment of inflammatory cells lead to the formation of crescents. The crescents can also be seen in other conditions like lupus nephritis, but they are less characteristic and not as frequent as in anti-GBM disease and ANCA-associated vasculitis.

**Why Each Wrong Option is Incorrect**
* **Option A:** Focal segmental glomerulosclerosis (FSGS) is a common cause of nephrotic syndrome, but it is not typically associated with crescent formation. FSGS is characterized by sclerosis and collapse of glomeruli.
* **Option B:** Membranous nephropathy is another cause of nephrotic syndrome and is characterized by thickening of the glomerular basement membrane due to immune complex deposition. However, it is not typically associated with crescent formation.
* **Option C:** IgA nephropathy (Berger's disease) is a common cause of recurrent hematuria and can progress to chronic kidney disease. However, it is not typically associated with crescent formation.

**Clinical Pearl / High-Yield Fact**
Crescent formation is a marker of severe glomerular injury and is associated with a poor prognosis. Early recognition and treatment of conditions leading to crescent formation are essential to prevent rapid progression to end-stage renal disease.

**Correct Answer:** D.