**Core Concept:** McArdle syndrome is a genetic disorder caused by a deficiency of the enzyme myophosphorylase, which is essential in the process of muscle glycogenolysis. This results in impaired breakdown of glycogen during exercise, leading to muscle fatigue, pain, and cramps.

**Why the Correct Answer is Right:** Myophosphorylase deficiency in McArdle syndrome results in impaired glycogen breakdown, particularly during intense and prolonged exercise. Without myophosphorylase, glycogen cannot be converted into its breakdown products, such as glucose-6-phosphate, thus hindering the replenishment of ATP in the muscle cells. This leads to reduced energy production during exercise, causing the characteristic symptoms of McArdle syndrome.

**Why Each Wrong Option is Incorrect:**

A. **Myophosphorylase deficiency is present in McArdle syndrome:** This is incorrect because McArdle syndrome is caused by a deficiency of myophosphorylase, not due to an excess of it.

B. **Increased glycogenolysis occurs in McArdle syndrome:** In fact, myophosphorylase deficiency impairs glycogenolysis, leading to reduced ATP production in muscles during exercise.

C. **Muscle strength is normal in McArdle syndrome:** Muscle weakness is a prominent feature of the condition, and is a consequence of impaired glycogen breakdown during exercise.

D. **Muscle cramps are absent in McArdle syndrome:** In reality, muscle cramps are a common symptom in McArdle syndrome, as a result of impaired glycogen breakdown and reduced ATP production during exercise.

**Clinical Pearl:** McArdle syndrome is a valuable reminder of the importance of proper enzyme function for maintaining muscle health and preventing fatigue during physical activity. It highlights the crucial role of myophosphorylase in glycogenolysis and the subsequent effects on ATP production in muscle cells.