A 25-year-old Japanese woman was brought to the Emergency Room on a stretcher as she was found lying on the floor of her apartment unable to move by her roommate. She had participated in a 5K race at a local park, the day before and had eaten a full chocolate cake before going to bed. She got up around 4 am to get a glass of water and noticed weakness in her limbs. She could hardly hold the glass and finally fell on the kitchen floor. Over the past few weeks, she has suffered from increasing heart palpitations and stress related to her upcoming exams.
Upon examination, she was of average build and anxious. She gave a clear account of the progression of her illness. Pulse was 110/min and irregularly irregular. Blood pressure was 130/90 mmHg. Cranial nerves were normal. Neck muscle power was 4/5. Shoulder girdle, truncal and pelvic girdle muscles 3/5. Tendon reflexes were decreased and plantar were absent. Sensation was intact.
What is the most probable diagnosis?