**Core Concept**
Whipple's disease is a rare, systemic bacterial infection caused by *Tropheryma whipplei*. It primarily affects the small intestine, leading to malabsorption and weight loss. The histopathological findings in Whipple's disease are characteristic and diagnostic.

**Why the Correct Answer is Right**
The characteristic histopathological finding in Whipple's disease is the presence of PAS (Periodic Acid-Schiff) positive macrophages containing rod-shaped bacilli in the lamina propria of the small intestine. This is due to the ingestion of *Tropheryma whipplei* by macrophages, which are then stained by PAS. The presence of these bacilli within macrophages is a hallmark of the disease. The lamina propria is the layer of loose connective tissue beneath the epithelial lining of the small intestine, and it is here that the macrophages containing the bacilli are typically found.

**Why Each Wrong Option is Incorrect**
**Option B:** Shortened, thickened villi with increased crypt depth are characteristic of celiac disease, not Whipple's disease. Celiac disease is an autoimmune disorder that causes the immune system to react to gluten, leading to inflammation and damage in the small intestine.
**Option C:** Blunting and flattening of the mucosal surface and absent villi are seen in conditions such as Crohn's disease or ulcerative colitis, which are types of inflammatory bowel disease. These changes are not characteristic of Whipple's disease.
**Option D:** Mononuclear infiltration at the base of crypts is a non-specific finding that can be seen in a variety of conditions, including infections and inflammatory disorders. It is not a diagnostic feature of Whipple's disease.

**Clinical Pearl / High-Yield Fact**
Whipple's disease should be considered in the differential diagnosis of malabsorption and weight loss, particularly in older adults. The diagnosis is often made through a combination of clinical presentation, histopathological findings, and PCR (polymerase chain reaction) testing for *Tropheryma whipplei*.

**✓ Correct Answer: A. PAS positive macrophages and rod shaped bacilli in lamina propria**