**Core Concept**
Lactose intolerance is a common condition where the body is unable to digest lactose, a sugar found in milk, due to a deficiency of lactase enzyme in the small intestine. This enzyme Breaks Down lactose into easily absorbed glucose and galactose.

**Why the Correct Answer is Right**
The correct answer is related to the pathophysiology of lactose intolerance. Lactase enzyme is responsible for breaking down lactose into glucose and galactose. When lactose is not broken down, it remains in the colon where it is fermented by bacteria, producing gas and causing symptoms such as bloating, abdominal cramps, and diarrhea. This is due to the inability of the small intestine to absorb lactose, leading to its accumulation in the colon.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because lactose intolerance is not caused by a deficiency of lactase enzyme in the pancreas. Lactase enzyme is produced in the small intestine, not the pancreas.

**Option B:** This option is incorrect because lactose intolerance is not caused by a genetic mutation that affects the production of lactose. While some people may have a genetic predisposition to lactose intolerance, it is not a direct result of a genetic mutation.

**Option C:** This option is incorrect because lactose intolerance is not caused by a deficiency of lactase enzyme in the stomach. Lactase enzyme is produced in the small intestine, not the stomach.

**Option D:** This option is incorrect because lactose intolerance is not caused by a bacterial overgrowth in the small intestine. While bacterial overgrowth can cause symptoms similar to lactose intolerance, it is not the underlying cause of the condition.

**Clinical Pearl / High-Yield Fact**
A simple way to remember the cause of lactose intolerance is to recall that it is a result of a deficiency of lactase enzyme in the small intestine, which is responsible for breaking down lactose into easily absorbed glucose and galactose.

**Correct Answer: B. Lactose intolerance is caused by a deficiency of lactase enzyme in the small intestine, which leads to an inability to break down lactose and causes symptoms such as bloating, abdominal cramps, and diarrhea.**