Mark is a 23-year-old graduate student who experienced flu-like symptoms for a week followed by a 1-month decline in his motor and sensory abilities. His respiratory function was even affected, requiring a ventilator. After this 1-month period, Mark began to recover his motor and sensory abilities. His physician believes he will make a full recovery. Which of the following conditions is the most likely? Explain why you chose this answer.

1. Multiple sclerosis

2. Guillain–Barré syndrome

3. Myasthenia gravis

4. Amyotrophic lateral sclerosis