## **Core Concept**
The question tests knowledge of vitamin deficiencies and their clinical manifestations, specifically focusing on gastrointestinal and renal symptoms. The correct answer involves identifying a vitamin deficiency that can cause both diarrhoea and renal stone formation. This requires understanding the metabolic roles and deficiency symptoms of various vitamins.

## **Why the Correct Answer is Right**
Vitamin D plays a crucial role in calcium absorption in the gut. A deficiency in Vitamin D can lead to decreased absorption of calcium, which in turn can cause an increase in parathyroid hormone (PTH) levels. Elevated PTH levels increase renal calcium excretion, which can lead to the formation of renal stones (nephrolithiasis). Additionally, Vitamin D deficiency can cause a range of gastrointestinal symptoms, although diarrhoea is not as directly linked as other symptoms like rickets in children or osteomalacia in adults. However, malabsorption associated with certain conditions can lead to diarrhoea.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because it does not specify a known vitamin deficiency directly linked to both diarrhoea and renal stone formation.
- **Option B:** Similarly, this option lacks specificity regarding a vitamin deficiency with the mentioned symptoms.
- **Option C:** This option is also incorrect as it does not directly correlate with the symptoms provided.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that Vitamin D deficiency can lead to increased risk of osteoporosis, osteomalacia, and rickets, and also has been linked to increased risks of various other conditions, including certain autoimmune diseases and diabetes. For renal stone formation and gastrointestinal symptoms like diarrhoea, clinicians should consider Vitamin D levels, especially in patients with malabsorption or those at risk for deficiency.

## **Correct Answer: C. Vitamin D.**