## Core Concept
The question pertains to a complication of severe infectious gastroenteritis, specifically a condition known as **lactose intolerance** or more accurately, a deficiency of an enzyme crucial for carbohydrate digestion. This condition arises due to damage to the intestinal mucosa, which affects the production of certain enzymes.

## Why the Correct Answer is Right
The correct answer, **lactase**, is an enzyme produced in the small intestine that breaks down lactose, a sugar found in milk and other dairy products, into glucose and galactose for absorption. Following an episode of severe infectious gastroenteritis, the intestinal mucosa can be damaged, leading to a temporary deficiency of **lactase**. This condition is known as **secondary lactose intolerance** or **acquired lactose deficiency**. It results in the inability to properly digest lactose, leading to symptoms such as bloating, diarrhea, and gas after consuming lactose-containing products.

## Why Each Wrong Option is Incorrect
- **Option A:** Not specified, but assuming it's an enzyme like amylase or sucrase, these are crucial for carbohydrate digestion but are not specifically associated with the development of intolerance following gastroenteritis in the same direct way lactase is.
- **Option B:** Similarly, not specified, but if it's another digestive enzyme, it would not directly relate to the specific scenario of lactose intolerance post-gastroenteritis.
- **Option C:** This option is not provided, but presumably, it could be another enzyme; however, without specifics, we acknowledge that lactase is uniquely implicated here.
- **Option D:** Not specified, but again, lactase is the key enzyme here.

## Clinical Pearl / High-Yield Fact
A critical point to remember is that **secondary lactose intolerance** is often temporary. The condition typically resolves once the underlying cause of the intestinal damage (like infectious gastroenteritis) is treated and the intestinal mucosa heals, which can take several weeks. Patients can often resume a normal diet, including lactose-containing products, once their symptoms improve.

## Correct Answer Line
**Correct Answer: B. lactase.**