A 20-year-old male presents to the clinic with a 4-month history of worsening speech slurring. He is feeling extremely embarrassed and is often mistaken for being drunk by his college friends. He is also experiencing excessive drooling and has noticed increasing weakness in lifting his right arm over the past 8 weeks. He has no significant medical history, does not smoke, and only drinks alcohol socially.

During the examination, he demonstrates weakness in raising his eyebrows, puffing his cheeks, and pursing his lips. He also displays significant scapula winging bilaterally with mild wasting of his right deltoid muscle. The musculature of his left arm is normal. The power of his right shoulder abduction and adduction is 3/5, with 5/5 in all other movements. Reflexes are all present with a normal sensory examination.

His blood tests reveal:

– Hb 142 g/l
– Platelets 390 * 109/l
– WBC 6.0 * 109/l
– Na+ 140 mmol/l
– K+ 4.5 mmol/l
– Urea 4.8 mmol/l
– Creatinine 60 ┬Ámol/l
– CRP 3 mg/l
– Creatine kinase 155 IU/l (50-335)
– TSH 2.5 mu/l
– HIV negative

What is the most likely diagnosis?