A 62-year-old man is evaluated in the emergency department for a 10-day history of progressively worsening shortness of breath, a 3-month history of difficulty with swallowing at dinnertime that resolves by the morning, and a 5-month history of intermittent blurry vision and fatigue. He had a urinary tract infection 2 weeks ago that was treated with ciprofloxacin, after which the fatigue and other symptoms markedly worsened.
On physical examination, temperature, blood pressure, and pulse rate are normal; respiration rate is 24/min. Other notable findings are bilateral ptosis, diplopia with sustained horizontal gaze, nasal speech, a snarling smile, weakness of cervical flexion, and symmetric weakness of shoulder abduction, arm extension, and hip flexion that becomes more severe with repeated effort. Neurologic examination shows an awake and oriented patient with normal sensation and deep tendon reflexes; no atrophy or fasciculation is noted.
Serum magnesium level is 1.5 mg/dL (0.62 mmol/L). Results of other laboratory studies, including a complete blood count and a comprehensive metabolic profile, are normal.
The patient is admitted to the ICU, where serial respiratory measurements reveal declining vital capacity and negative inspiratory force.
Which of the following is the most appropriate emergent treatment?