A 28-year-old woman presents to the emergency room with complaints of shortness of breath and intermittent double vision. While her shortness of breath started two days ago, her double vision has been progressively worsening over the last one month. She works as a teacher and the symptoms increase the longer she works on the black board, worse by the evening but relieved with rest. She has also noted drooping of her eyelids, which seems to worsen with prolonged working at the school. A detailed review of systems reveals some difficulty eating, change in voice quality, and weakness in chewing food. Physical examination discloses bilateral ptosis of both eyes that worsens when the patient is asked to actively keep her eyelids elevated. Her lung examination revealed bilateral wheezing and tachypnea. She has limb and trunk weakness in a proximal greater than distal distribution. Her symptoms markedly improved after edrophonium administration. Ice pack test turned out to be positive. Of the following, which is the most effective agent to treat this disorder?
