Anna and Beth have decided to run the Great Ocean Road Marathon this year. Since they have never run a marathon before and don’t usually exercise much, they decide to start practicing early on in preparation.  

They go for a run one early morning and for the first 2-3 minutes everything goes well, but Beth suddenly feels intense cramping and pain in her muscles. They both stop and after resting for about 5 minutes Beth starts to feel better and starts running again.

The same thing happens each time they go for a run and Beth decides to visit a doctor. The doctor diagnoses her with McArdle’s disease and recommends she avoid strenuous exercise.

This disease is caused by a deficiency in the glycogen phosphorylase enzyme which is involved in glycogen breakdown.

Which glycogen phosphorylase is affected?