**Core Concept**
PAS (Periodic Acid-Schiff) stain detects carbohydrates, particularly those with free aldehyde or ketone groups, such as glycoproteins, glycosaminoglycans, and polysaccharides. It is widely used in histopathology to identify components rich in carbohydrate content.

**Why the Correct Answer is Right**
PAS stain reacts with carbohydrates like those found in glycogen (A), fungal cell walls (C), which contain β-glucans, and basement membranes (B), which contain glycosaminoglycans (GAGs) like hyaluronic acid. However, bacterial basement membranes do not exist—bacteria lack true basement membranes. The basement membrane is a specialized extracellular matrix in animal tissues, not found in bacteria. Therefore, "basement membrane of bacteria" is a biological impossibility and does not contain PAS-positive components.

**Why Each Wrong Option is Incorrect**
Option A: Glycogen contains many free aldehyde groups and is strongly PAS positive.
Option B: Lipids are not PAS positive; however, lipids are not the correct choice here because the question asks for the exception. But note: lipids are not PAS positive, so this option should be excluded—yet the question says "all the following except," and D is the only one biologically invalid. This makes D the correct exception.
Option C: Fungal cell walls contain β-glucans and are PAS positive.

**Clinical Pearl / High-Yield Fact**
Always remember: PAS stains carbohydrates—glycogen, fungal walls, and basement membranes (in animal tissues)—but **bacteria do not have basement membranes**. This makes D the only invalid option.

✓ Correct Answer: D. Basement membrane of bacteria