**Question:** McArdle's maximum allowable sweat rate is

A. 0.5 liters/hour
B. 1.0 liters/hour
C. 1.5 liters/hour
D. 2.0 liters/hour

**Core Concept:** McArdle's disease is a genetic disorder affecting the skeletal muscles, characterized by a deficiency of the enzyme phosphofructokinase (PFK) in the glycolytic pathway. The disease results in decreased ability to produce ATP during intense exercise, leading to muscle cramps, fatigue, and pain. The maximal sweat rate is an indicator of the body's ability to cool itself during physical exertion, which is crucial for preventing heat exhaustion and heatstroke, especially in hot and humid conditions.

**Why the Correct Answer is Right:** The correct answer, **A. 0.5 liters/hour**, reflects the maximum sweat rate that a healthy individual can produce under normal conditions. In McArdle's disease, the ability to produce ATP during intense exercise is impaired. As a result, the body cannot produce enough sweat to cool itself effectively during physical exertion. This leads to an increased risk of heat exhaustion and heatstroke. Therefore, McArdle's patients may not be able to reach the same maximal sweat rate as healthy individuals.

**Why Each Wrong Option is Incorrect:**

**Option B (1.0 liters/hour)** is too high for a healthy person and may be achieved by individuals who are well-hydrated and have adequate sweat glands. However, in McArdle's patients, this rate is not attainable due to impaired ATP production.

**Option C (1.5 liters/hour)** is also too high for a healthy individual and may be achieved by individuals with higher body surface area or more active lifestyles. However, McArdle's patients would still struggle to achieve this rate due to their ATP production limitations.

**Option D (2.0 liters/hour)** is a very high sweat rate that may be seen in individuals participating in intense endurance sports. While this rate is not achievable for a healthy person, it is even more unrealistic for a patient with McArdle's disease, who would face challenges in producing sufficient sweat to cool down effectively.

**Clinical Pearls:** The ability to produce enough sweat is crucial for preventing heat exhaustion and heatstroke. A maximal sweat rate of 0.5 liters/hour is considered appropriate for healthy individuals, while a rate of 1.0 liters/hour is acceptable for individuals with higher body surface area or more active lifestyles. However, these rates are still inadequate for patients with McArdle's disease. Achieving high sweat rates is particularly challenging for these patients due to their impaired ATP production during intense exercise.