A 28-year-old woman from Delhi presents with a 3-month history of progressive ptosis and diplopia. Symptoms worsen towards the evening and improve after rest. On examination, she has bilateral ptosis that worsens with sustained upward gaze for 30 seconds. Pupils are normal and reactive. She has mild proximal weakness of the shoulders and hips. Serum acetylcholine receptor (AChR) antibodies are positive. Chest CT shows no thymoma. Which of the following is the most appropriate initial pharmacological management?