This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Genetics, Subject name: Medicine
MCardle's syndrome is due to 
Select and explain. 
A. Deficiency of glucose-6-phosphatase, B. Absence of muscle phosphorylase, C. Deficiency of liver phosphorylase, D. Deficiency of liver phosphorylase kinase
A: Deficiency of glucose-6-phosphatase
Answer: A: Deficiency of glucose-6-phosphatase 
Explanation: McArdle disease (also known as glycogen storage disease type V) is a disorder affecting muscle metabolism. The condition is caused by the lack of an enzyme called muscle phosphorylase. This results in an inability to break down glycogen 'fuel' stores. McArdle disease leads to pain and fatigue with strenuous exercise. Ref Robbins 9/e pg 153