A 19-year-old man presents to the clinic complaining of early fatigue and muscle cramps while playing sports. He is fine when walking or doing less intense levels of work. On physical examination, he appears well and the vital signs are normal. Muscle bulk, tone and strength in the proximal muscles are normal. There is no muscle fatigue with repetitive arm grip exercises. After an exercise stress test, his serum creatine kinase (CK) is elevated and lactate level is normal. Which of the following is the most likely diagnosis?

**Core Concept**
McArdle disease, also known as glycogen storage disease type V, is a genetic disorder caused by a deficiency of the enzyme myophosphorylase. This enzyme is crucial for the breakdown of glycogen in skeletal muscles, leading to impaired muscle energy production and exercise intolerance.

**Why the Correct Answer is Right**
The patient's symptoms of early fatigue and muscle cramps during intense exercise, normal muscle strength and tone, and elevated serum creatine kinase (CK) levels are characteristic of McArdle disease. The normal lactate level during exercise stress testing is particularly indicative of this condition, as patients with McArdle disease are unable to produce lactic acid due to their impaired glycogen breakdown. This is because myophosphorylase is necessary for the initial step of glycogenolysis, which generates glucose-1-phosphate that is then converted to glucose-6-phosphate and eventually to fructose-1,6-bisphosphate, producing energy through glycolysis. Without myophosphorylase, this process is blocked, and lactic acid production is impaired.

**Why Each Wrong Option is Incorrect**
**Option A:** Gaucher disease is a lysosomal storage disorder caused by a deficiency of glucocerebrosidase, leading to accumulation of glucocerebroside in macrophages. It does not typically present with muscle cramps or exercise intolerance.

**Option B:** Tay-Sachs disease is a genetic disorder caused by a deficiency of hexosaminidase A, leading to accumulation of GM2 ganglioside in neurons. It presents with neurodegenerative symptoms, not muscle cramps or exercise intolerance.

**Option D:** Hemochromatosis is a genetic disorder causing excessive iron absorption, leading to iron overload and tissue damage. It can cause muscle weakness and fatigue, but the presentation is typically more chronic and not specifically related to exercise intolerance.

**Clinical Pearl / High-Yield Fact**
McArdle disease is a classic example of a primary muscle disorder, where the defect is confined to the muscle itself, rather than a systemic disease affecting multiple organs. This is in contrast to secondary muscle disorders, where the defect is caused by a systemic disease or condition.

**✓ Correct Answer:** C. McArdle disease (glycogen storage disease)

✓ Correct Answer: C. McArdle disease (glycogen storage disease)