**Core Concept**
Glycogen storage disorders (GSDs) are a group of metabolic disorders caused by defects in enzymes involved in glycogen synthesis, breakdown, or storage. These disorders lead to abnormal glycogen accumulation in various tissues, resulting in diverse clinical manifestations.

**Why the Correct Answer is Right**
The correct answer involves the enzyme glycogen synthase, which is responsible for glycogen synthesis. In GSD type 0, also known as glycogen synthase deficiency, the enzyme glycogen synthase is deficient or defective, leading to impaired glycogen synthesis and accumulation of abnormal glycogen in the liver and muscles. This results in hypoglycemia, lactic acidosis, and hepatomegaly.

**Why Each Wrong Option is Incorrect**
**Option A:** Glycogen storage disorder type I (GSD I) is caused by a deficiency of glucose-6-phosphatase, not glycogen synthase.
**Option B:** McArdle's disease is a type of glycogen storage disorder caused by a deficiency of muscle glycogen phosphorylase, not glycogen synthase.
**Option C:** This option is incorrect as there is no widely recognized glycogen storage disorder caused by a deficiency of glycogen synthase.

**Clinical Pearl / High-Yield Fact**
Glycogen storage disorders can be classified into two main groups: those that result in glycogen accumulation (e.g., GSD type I, McArdle's disease) and those that result in glycogen deficiency (e.g., GSD type 0).

**Correct Answer: C. Glycogen synthase deficiency.**