**Core Concept**
Glucose-6-phosphatase deficiency is a key enzyme deficiency that leads to glycogen storage diseases, specifically type Ia. This enzyme is crucial for glucose homeostasis and the breakdown of glycogen to glucose.

**Why the Correct Answer is Right**
Glucose-6-phosphatase deficiency results in the accumulation of glycogen in the liver and kidneys. This is because glucose-6-phosphatase catalyzes the final step in glycogenolysis, converting glucose-6-phosphate to glucose, which can then be released into the bloodstream. Without this enzyme, glycogen remains stored, and glucose levels cannot be adequately regulated. The deficiency of glucose-6-phosphatase is inherited in an autosomal recessive manner, making it one of the most common enzyme deficiencies.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify a particular enzyme deficiency. There are many enzyme deficiencies, each with distinct clinical manifestations and pathophysiology.
**Option B:** This option is incorrect because lactase deficiency is a separate condition that affects the digestion of lactose, a sugar found in milk. It is not related to glycogen storage or glucose homeostasis.
**Option C:** This option is incorrect because it is not a well-known enzyme deficiency. While there are many enzyme deficiencies that affect glycogen metabolism, glucose-6-phosphatase deficiency is the most common.

**Clinical Pearl / High-Yield Fact**
Glucose-6-phosphatase deficiency can lead to severe hypoglycemia, particularly in the fasting state. This is because the body is unable to break down glycogen to glucose, leading to low blood glucose levels.

**Correct Answer:** A. Glucose-6-phosphatase deficiency.