Okay, let's tackle this question. The user is asking about a glycogen storage disorder caused by myophosphorylase deficiency. First, I need to recall the different types of glycogen storage diseases (GSDs) and their associated enzymes.

Glycogen storage diseases are categorized by type, each related to a specific enzyme deficiency. Myophosphorylase is an enzyme involved in glycogen breakdown in muscle cells. The key here is to remember which GSD is linked to this enzyme.

I think McArdle's disease is the one associated with myophosphorylase deficiency. That's GSD type V. Let me verify. Yes, GSD V is caused by a lack of myophosphorylase in skeletal muscles, leading to exercise intolerance and muscle cramps because the muscle can't break down glycogen for energy during activity.

Looking at the options, the correct answer would be the one labeled as GSD type V or McArdle's disease. The other options are different types of GSDs. For example, Pompe disease is GSD type II (acid alpha-glucosidase deficiency), von Gierke's is type I (glucose-6-phosphatase), and Cori's disease is type III (debranching enzyme). Each of these affects different enzymes and tissues, so they present with different symptoms.

The clinical pearl here is that McArdle's disease is the only GSD where myophosphorylase is deficient in muscle. Students should remember that GSD types are classified by enzyme deficiency and affected tissue. Also, the hallmark symptoms like exercise-induced muscle pain and positive phosphorus uptake on MRI can help in diagnosis.

**Core Concept** Glycogen storage disorders (GSDs) are inherited metabolic diseases caused by enzyme deficiencies in glycogen metabolism. Myophosphorylase deficiency specifically affects glycogen breakdown in skeletal muscle, leading to exercise intolerance and muscle cramps.

**Why the Correct Answer is Right** McArdle disease (GSD type V) results from a deficiency of myophosphorylase in skeletal muscle. This enzyme catalyzes the initial step of glycogenolysis in muscle, breaking glycogen into glucose-1-phosphate. Its absence prevents muscle cells from utilizing glycogen for energy during anaerobic activity, causing exercise-induced cramps, myoglobinuria, and second-wind phenomenon.

**Why Each Wrong Option is Incorrect**
**Option A:** Pompe disease (GSD type II) is due to acid α-glucosidase deficiency, leading to lysosomal glycogen accumulation in heart and skeletal muscle.
**Option B:** von Gierke disease (GSD type I) involves glucose-6-phosphatase deficiency, affecting liver and kidneys, causing hypoglycemia and hepatomegaly.
**Option C:** Cori disease (GSD type III) stems from amylo-1,6-glucosidase (debrancher) deficiency, causing hepatic and muscle involvement with mild myopathy.

**Clinical Pearl** McArdle disease is the only GSD where myophosphorylase deficiency is confined to skeletal muscle. Diagnosis is confirmed by muscle biopsy showing "dystrophic glycogen" and elevated phosphate uptake on MRI after exercise.

**Correct Answer: C. McArdle disease (Glycogen Storage Disease Type

✓ Correct Answer: A. McArdle's disease.