**Core Concept**
Fructose intolerance, also known as essential fructosuria, is a benign condition where individuals have a deficiency in the enzyme fructokinase. This enzyme is responsible for phosphorylating fructose to fructose-1-phosphate, which is then further metabolized by aldolase B. The deficiency leads to the accumulation of fructose in the blood.

**Why the Correct Answer is Right**
Fructose intolerance is caused by a deficiency in the enzyme fructokinase, which is encoded by the **fructokinase (FRK) gene**. This enzyme catalyzes the initial step in fructose metabolism, converting fructose into fructose-1-phosphate. Without sufficient fructokinase activity, fructose builds up in the blood, leading to symptoms such as nausea, vomiting, and abdominal pain after consuming fructose-containing foods. The deficiency is usually inherited in an autosomal recessive pattern.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to fructose intolerance. Lactase deficiency is associated with lactose intolerance, not fructose intolerance.
**Option B:** This option is incorrect because aldolase B deficiency leads to hereditary fructose intolerance, a more severe condition than essential fructosuria. Aldolase B is responsible for breaking down fructose-1-phosphate, and its deficiency leads to the accumulation of toxic intermediates.
**Option C:** This option is incorrect because glucose-6-phosphatase deficiency leads to glycogen storage disease type I, also known as von Gierke disease. This condition is related to glucose metabolism, not fructose intolerance.

**Clinical Pearl / High-Yield Fact**
It's essential to note that fructose intolerance is a benign condition, and patients with essential fructosuria usually do not require specific treatment. However, individuals with hereditary fructose intolerance, caused by aldolase B deficiency, require strict dietary avoidance of fructose-containing foods to prevent severe symptoms.

**Correct Answer: A. Fructokinase**