4th March 2024

A 24-year-old female presents to the rheumatology clinic with myalgia

A 24-year-old female presents to the rheumatology clinic with myalgia. She describes worsening myalgia with weakness and tiredness when carrying out various activities, including lifting and running. This has affected her upper limbs and lower limbs equally. On walking and running, she finds that her myalgia and tiredness temporarily improve after approximately 15 minutes of activity but would gradually worsen again as the activity progresses.

Neurological examination was unremarkable. No apparent muscle wasting is noted. Blood tests show an elevated CK level measuring 3500IU/L. The autoimmune screen was unremarkable. A muscle biopsy has shown periodic acid Schiff-positive vacuoles within the muscle tissue with an absence of myophosphorylase.

Which of the following likely represents the underlying diagnosis?

Select the SINGLE most appropriate option from the list below. Select ONE option only.