A 25-year-old female presented to the Emergency department with a five-day history of progressive weakness and lethargy. She initially complained of a constant dry mouth and nausea with some swallowing difficulties. Over the last few days, she has experienced blurring of vision and upper limb weakness, as well as breathlessness on exertion. She also reported a sore throat and abdominal pain, which she attributed to her period. The patient denied any medical problems but was a smoker of 20 cigarettes per day.
On examination, the patient was alert and orientated with evidence of intravenous drug abuse. Cranial nerve examination revealed mild bilateral ptosis, restricted ocular movements on right lateral and right superior gaze, reduced palatal movements, and weak neck extension. Upper limb examination showed reduced reflexes bilaterally with some proximal weakness, while lower limb examination showed depressed reflexes with normal power and sensory examination. Chest examination was normal, but abdominal examination was not performed.
A full blood count and biochemical profile were normal. Lumbar puncture showed an opening pressure of 12 cm H2O (5-18), protein of 0.65 g/L (0.15-0.45), white cell count of 3 per ml (≤5), and red cell count of 2 per ml (≤5). The Tensilon test was positive.
What is the likely diagnosis for this 25-year-old female patient?