On kidney biopsy, PAS-positive structures are:
**Core Concept**
The question is testing the ability to identify the correct cellular component in a kidney biopsy that stains positive for Periodic Acid-Schiff (PAS), a histological stain used to detect glycogen, glycoproteins, and mucins. PAS positivity is often associated with cellular components rich in carbohydrate groups.
**Why the Correct Answer is Right**
The correct answer is **D. Mesangial cells**. Mesangial cells are specialized cells in the glomerulus that produce and secrete mesangial matrix, which is rich in carbohydrate groups and stains positive with PAS. The PAS stain highlights the presence of glycogen and glycoproteins in these cells, making them distinct from other cellular components in the kidney biopsy. The presence of PAS-positive mesangial cells can be seen in various glomerular diseases, such as diabetic nephropathy and membranoproliferative glomerulonephritis.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because **Epithelial cells** in the kidney are not typically PAS-positive. While some epithelial cells may contain glycogen, they do not exhibit the same level of PAS positivity as mesangial cells.
**Option B:** This option is incorrect because **Endothelial cells** in the kidney are not typically PAS-positive. Endothelial cells do contain some glycoproteins, but they are not as rich in carbohydrate groups as mesangial cells.
**Option C:** This option is incorrect because **Tubular cells** in the kidney are not typically PAS-positive. While some tubular cells may contain glycogen, they do not exhibit the same level of PAS positivity as mesangial cells.
**Clinical Pearl / High-Yield Fact**
When interpreting kidney biopsies, it's essential to note that PAS positivity can be seen in various glomerular diseases, including diabetic nephropathy and membranoproliferative glomerulonephritis. The presence of PAS-positive mesangial cells can be a useful diagnostic clue in these conditions.
**Correct Answer:** D. Mesangial cells.