## **Core Concept**
The question tests the understanding of pediatric gastroenterology, specifically the diagnosis of conditions leading to chronic diarrhea in infants and toddlers. The key details provided are the acidic nature of the stools (pH < 5.5) and the presence of reducing substances in the stools. These findings are critical for differentiating between types of diarrhea. ## **Why the Correct Answer is Right** The presence of acidic stools (pH < 5.5) and reducing substances in the stools points towards a diagnosis of **lactose intolerance** or more specifically in this context, **lactose malabsorption**. This condition occurs when the small intestine does not produce enough lactase, the enzyme that breaks down lactose (a sugar found in milk and other dairy products) into glucose and galactose. As a result, lactose remains undigested and acts as an osmotic agent, drawing water into the intestine and causing diarrhea. The undigested lactose can also be fermented by colonic bacteria, producing short-chain fatty acids and lactic acid, which contribute to the acidic pH of the stools. The presence of reducing substances (which lactose is) in the stools supports this diagnosis. ## **Why Each Wrong Option is Incorrect** - **Option A:** This option is not provided, but typically, conditions like celiac disease or cystic fibrosis could present with chronic diarrhea. However, they would not specifically cause acidic stools with reducing substances. - **Option B:** Similarly, not provided, but other causes of diarrhea like infectious gastroenteritis or inflammatory bowel disease might not specifically result in the combination of acidic stools and presence of reducing substances. - **Option D:** Without the specific details of Option D, it's hard to directly refute it, but generally, conditions not related to carbohydrate malabsorption would not present with these specific stool characteristics. ## **Clinical Pearl / High-Yield Fact** A key clinical pearl is that the presence of **reducing substances** in the stool can help differentiate between osmotic and secretory diarrhea. Reducing substances are substances that can act as reducing agents, and their presence suggests an osmotic cause, such as carbohydrate malabsorption. A simple test for reducing substances can be done using Benedict's reagent. This finding, combined with an acidic stool pH, strongly suggests a diagnosis of carbohydrate malabsorption, such as lactose intolerance. ## **Correct Answer:** C.