Periodic acid schiff stain shows Block positivity in –
**Core Concept**
The Periodic Acid-Schiff (PAS) stain is a histological technique used to detect carbohydrates, glycogen, and glycoproteins in tissues. Block positivity refers to a specific staining pattern where the PAS stain accumulates in a block-like or diffuse manner within the cytoplasm of cells.
**Why the Correct Answer is Right**
Block positivity in PAS stain is characteristic of glycogen storage diseases, particularly in the case of McArdle's disease (Glycogen storage disease type V). In this condition, there is a deficiency of the enzyme myophosphorylase, which leads to the accumulation of glycogen in the cytoplasm of muscle cells. The PAS stain reacts with the glycogen, resulting in a block-like positivity. This is a diagnostic feature of McArdle's disease.
**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because PAS block positivity is not typically associated with other glycogen storage diseases, such as von Gierke's disease (Glycogen storage disease type I).
**Option B:** Incorrect because PAS block positivity is not a characteristic feature of Pompe disease (Glycogen storage disease type II), which is caused by a deficiency of acid alpha-glucosidase.
**Option C:** Incorrect because PAS block positivity is not a specific feature of other muscle diseases, such as muscular dystrophy.
**Clinical Pearl / High-Yield Fact**
PAS block positivity can be a valuable diagnostic clue in the identification of glycogen storage diseases, particularly in the context of muscle biopsy.
**Correct Answer:** D. McArdle's disease