MRCP2-3427

A 23-year-old male presents to the clinic after being referred by his GP. He has been noticing that his eyelids appear to be drooping more and more over the past 10 months, which has been commented on by his friends. He denies experiencing any muscle weakness or double vision. He has no significant medical history and does not smoke or drink alcohol. Upon examination, bilateral abduction and adduction of his eyes are impaired, and there is inconsistent impairment of vertical gaze. His neurological examination of the upper and lower limbs is unremarkable, except for mild finger-nose dysmetria. Blood tests show no abnormalities, and his ECG shows sinus rhythm with a PR interval of 260 ms. Fundoscopy reveals a pigmented retina. What is the likely diagnosis?