**Core Concept**
The patient's presentation with diarrhoea, poor appetite, and malabsorption, along with histological findings of crypt hyperplasia, villi atrophy, and infiltration of CD8+ T cells in the epithelium, suggests a form of small intestine pathology characterized by immune-mediated damage to the intestinal mucosa.

**Why the Correct Answer is Right**
The presence of CD8+ T cells in the epithelium is a hallmark of autoimmune enteropathy, specifically celiac disease. The immune response against gluten in individuals with celiac disease leads to the activation of CD8+ T cells, which then cause damage to the intestinal epithelium. This results in villi atrophy and crypt hyperplasia, impairing nutrient absorption and leading to symptoms such as diarrhoea and malabsorption.

**Why Each Wrong Option is Incorrect**
**Option A:** Crohn's disease typically presents with granulomatous inflammation and is less likely to show CD8+ T cell infiltration.
**Option B:** Ulcerative colitis primarily affects the colon and is characterized by neutrophilic infiltration, not CD8+ T cells.
**Option C:** Infections such as giardiasis may cause malabsorption but do not typically lead to villi atrophy or CD8+ T cell infiltration.
**Option D:** Lactose intolerance is a common cause of diarrhoea but does not result in the histological changes described.

**Clinical Pearl / High-Yield Fact**
Celiac disease is a classic example of an immune-mediated disorder that can be triggered by dietary exposure to gluten. A thorough history of dietary exposure is crucial in diagnosing this condition.

**Correct Answer:** C.