### Core Concept

Diarrhoea is a common gastrointestinal symptom that can be caused by various factors, including medications, infections, and metabolic disorders. The pathophysiology of diarrhoea involves the abnormal movement of water and electrolytes into the intestinal lumen, leading to a loose or watery stool.

### Why the Correct Answer is Right

The correct answer is **Loperamide**. Loperamide is an opioid receptor agonist that primarily acts on the mu-opioid receptors in the intestinal mucosa. By activating these receptors, loperamide slows down the movement of intestinal contents, increases the absorption of water and electrolytes, and decreases the secretion of fluid into the intestinal lumen. This results in a decrease in stool frequency and consistency.

### Why Each Wrong Option is Incorrect

**Option A:** **Castor oil** can cause diarrhoea due to its laxative effect, which is mediated by the release of prostaglandins and other bioactive lipids that stimulate intestinal motility and secretion.

**Option B:** **Metronidazole** can cause diarrhoea as a side effect, possibly due to its effect on the intestinal microbiota or its direct action on the intestinal epithelium.

**Option C:** **Magnesium sulphate** can cause diarrhoea due to its osmotic effect, which draws water into the intestinal lumen and increases stool frequency and fluidity.

### Clinical Pearl / High-Yield Fact

Loperamide is often used to treat diarrhoea caused by various conditions, including gastroenteritis, inflammatory bowel disease, and irritable bowel syndrome. However, it is essential to note that loperamide can cause constipation if taken in excess or for prolonged periods.

### Correct Answer: D. Loperamide