A 32-year-old woman presents to the neurology clinic with progressive muscle weakness, ptosis, and diplopia that worsens towards the end of the day. Repetitive nerve stimulation shows a decremental response. Serology is positive for anti-acetylcholine receptor (AChR) antibodies. She is started on pyridostigmine but develops severe abdominal cramping and diarrhea within hours. What is the most appropriate immediate next step in management?