**Core Concept**
The Periodic Acid Schiff (PAS) stain is a histochemical technique used to detect carbohydrate-containing molecules, such as glycogen, glycoproteins, and glycolipids, in tissue sections. In the context of the gastrointestinal tract, PAS-positive macrophages in the lamina propria are indicative of a specific condition.

**Why the Correct Answer is Right**
PAS-positive macrophages in the lamina propria are characteristic of Whipple's disease, a rare, systemic bacterial infection caused by *Tropheryma whipplei*. The lamina propria, a layer of loose connective tissue in the gastrointestinal tract, harbors these PAS-positive macrophages, which contain the bacteria. The presence of these macrophages, along with the patient's symptoms of abdominal pain and diarrhea, points towards Whipple's disease.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not match the description of PAS-positive macrophages in the lamina propria, which is a hallmark of Whipple's disease.
* **Option B:** This option is incorrect because it does not describe a condition associated with PAS-positive macrophages in the gastrointestinal tract.
* **Option C:** This option is incorrect because it does not match the description of PAS-positive macrophages in the lamina propria, which is a characteristic of Whipple's disease.

**Clinical Pearl / High-Yield Fact**
Whipple's disease is a rare, systemic bacterial infection that requires prompt diagnosis and treatment to prevent complications. The presence of PAS-positive macrophages in the lamina propria is a key diagnostic feature, and a biopsy of the small intestine is often performed to confirm the diagnosis.

**Correct Answer:** D. Whipple's disease