**Core Concept**
Muscle cramps and weakness with exercise in the context of glycogen accumulation on muscle biopsy suggest a disorder of glycogen metabolism. This condition is characterized by the inability to break down glycogen to glucose for energy production, leading to its accumulation in muscles.

**Why the Correct Answer is Right**
The most likely diagnosis is McArdle's disease, also known as glycogen storage disease type V. This condition is caused by a deficiency of the enzyme myophosphorylase, which is necessary for the breakdown of glycogen to glucose-1-phosphate in muscles. The accumulation of glycogen leads to muscle cramps, weakness, and fatigue with exercise. The fact that the hepatic biopsy is unremarkable suggests that the enzyme deficiency is specific to muscles.

**Why Each Wrong Option is Incorrect**
**Option A:** Pompe disease, caused by a deficiency of acid alpha-glucosidase, typically presents with cardiomegaly and muscle weakness, but the glycogen accumulation is more prominent in the liver.

**Option B:** Cori's disease, also known as glycogen storage disease type III, is caused by a deficiency of the debranching enzyme. However, it typically presents with hepatomegaly and hypoglycemia, rather than muscle cramps and weakness.

**Option C:** Hers' disease, also known as glycogen storage disease type VI, is caused by a deficiency of liver phosphorylase. It typically presents with hypoglycemia and hepatomegaly, rather than muscle cramps and weakness.

**Option D:** Forbe's disease, also known as glycogen storage disease type IIIb, is a variant of Cori's disease. It typically presents with hepatomegaly and hypoglycemia, rather than muscle cramps and weakness.

**Clinical Pearl / High-Yield Fact**
McArdle's disease is an autosomal recessive disorder, and patients often have a family history of similar symptoms. It is essential to consider this diagnosis in young patients with exercise-induced muscle cramps and weakness.

**Correct Answer:** C.