A 52-year-old woman with myasthenia gravis (MG) is on pyridostigmine 60 mg four times daily. She presents to the clinic with progressive weakness, ptosis, and diplopia over the past 3 days despite compliance with her medication. On examination, she has a respiratory rate of 24/min, weak voice, and difficulty swallowing. A single-dose edrophonium (Tensilon) test is performed: within 30 seconds, her symptoms dramatically worsen. Which of the following best explains this clinical response and guides the next therapeutic step?