**Diagnostic Dilemma**

**Core Concept**
The patient's presentation suggests a chronic condition affecting the small intestine, with recent onset of pruritus and elevated alkaline phosphatase, indicating a possible connection to the enterohepatic circulation of bile salts.

**Why the Correct Answer is Right**
The patient's history of long-standing diarrhoea, recent onset of pruritus, and elevated alkaline phosphatase levels in the absence of biliary tract abnormalities or liver dysfunction points towards a diagnosis of **Primary Biliary Cholangitis (PBC)**. PBC is an autoimmune disease characterized by the progressive destruction of intrahepatic bile ducts, leading to cholestasis and elevated levels of alkaline phosphatase. The enterohepatic circulation of bile salts is impaired, resulting in the absorption of toxic bile salts in the small intestine, leading to pruritus.

**Why Each Wrong Option is Incorrect**
* **Option A:** **Celiac disease** typically presents with abdominal pain, diarrhea, and weight loss, but it does not typically cause pruritus or elevated alkaline phosphatase.
* **Option B:** **Crohn's disease** can cause chronic diarrhea, malabsorption, and weight loss, but it is not characteristically associated with pruritus or elevated alkaline phosphatase.
* **Option C:** **Bile duct cancer** would typically present with jaundice, weight loss, and abdominal pain, rather than isolated pruritus and elevated alkaline phosphatase.

**Clinical Pearl / High-Yield Fact**
PBC is often associated with other autoimmune diseases, such as Sjögren's syndrome, thyroid disease, and rheumatoid arthritis, emphasizing the importance of a thorough autoimmune workup in patients with PBC.

**Correct Answer: D. Primary Biliary Cholangitis**