## **Core Concept**
The question revolves around the diagnosis of a condition causing persistent diarrhea in a two-year-old child, characterized by acidic stools and the presence of a reducing substance. This clinical presentation suggests a disorder of carbohydrate digestion or absorption.

## **Why the Correct Answer is Right**
The presence of reducing substances in the stool along with acidic stools points towards a condition where there is an issue with carbohydrate digestion. The most common cause of such a presentation is **lactose intolerance** or more specifically in the context of the question, **sucrase-isomaltase deficiency**, a type of congenital sucrase-isomaltase deficiency. This enzyme deficiency leads to an inability to digest sucrose and certain other carbohydrates, resulting in osmotic diarrhea and the presence of reducing sugars in the stool. The acidic nature of the stools is due to the fermentation of undigested carbohydrates by colonic bacteria, producing short-chain fatty acids.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is not provided, but typically, conditions not related to carbohydrate malabsorption or with different clinical presentations would be incorrect.
- **Option B:** Similarly, without the specific option, we can infer that any condition not directly related to the inability to digest carbohydrates (like pancreatic insufficiency, celiac disease, or other malabsorption syndromes) would not fit the specific clues given (reducing substances and acidic stools).
- **Option C:** Again, without specifics, any condition that does not directly correlate with carbohydrate digestion issues (e.g., infectious diarrhea, inflammatory bowel disease) would not be the best fit.
- **Option D:** Assuming this is not the correct answer, it would likely represent another diagnosis not directly linked to the specific findings of reducing substances in acidic stools due to carbohydrate malabsorption.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that in cases of suspected carbohydrate malabsorption (like in sucrase-isomaltase deficiency), a **dietary trial** with a carbohydrate-restricted diet or a specific diet lacking the suspected problematic carbohydrate can be both diagnostic and therapeutic. Additionally, the presence of **reducing substances** in the stool can be tested using Benedict's solution, which turns green, yellow, orange, or red-brick color depending on the concentration of the reducing sugar.

## **Correct Answer:** D. Sucrase Isomaltase deficiency.