**Core Concept**
Myophosphorylase deficiency, also known as McArdle's disease, is a genetic disorder characterized by the deficiency of myophosphorylase enzyme in the muscle tissue. This enzyme is crucial for the breakdown of glycogen to glucose-1-phosphate, which is then used as an energy source during muscle contractions.

**Why the Correct Answer is Right**
The deficiency of myophosphorylase leads to the accumulation of glycogen in the muscle cells, resulting in muscle cramps, weakness, and fatigue after exercise. This is because the muscle cells are unable to break down glycogen to glucose-1-phosphate, leading to a decrease in energy availability. The affected individuals often experience a sudden onset of muscle cramps and weakness within 10-15 minutes of exercise, a phenomenon known as "second wind" or "exercise-induced fatigue".

**Why Each Wrong Option is Incorrect**
**Option A:** Muscle weakness at rest is not a characteristic feature of McArdle's disease.
**Option B:** The condition is not caused by a deficiency of lactate dehydrogenase.
**Option C:** McArdle's disease is not associated with an increased risk of cardiac arrhythmias.

**Clinical Pearl / High-Yield Fact**
McArdle's disease is often referred to as the "exercise-induced fatigue" syndrome, and it is essential to note that the symptoms typically occur within 10-15 minutes of exercise and are relieved by rest.

**Correct Answer:** C.