A 6-Month-Old Boy Is Brought to the Emergency Department Due

A 6-month-old boy is brought to the emergency department due to poor feeding.  His mother says, "My son normally nurses every 3 hours, but for the past 24 hours, he hasn't been latching or sucking when I offer him the breast.  A few days ago, his eyelids started to appear droopy, and they don't open fully today.  He also was previously able to roll and sit by himself, but today he is only lying down and not doing anything."  The infant normally stools 2-3 times per day but has not stooled for 7 days, despite the family giving him small amounts of prune juice.  He was born at 40 weeks gestation by vaginal delivery after an uncomplicated pregnancy.  Immunizations are up to date, and he takes no medications.  He lives on a large rural farm with his parents and grandfather.  Temperature is 36.7 C (98 F) , pulse is 128/min, respirations are 30/min, and pulse oximetry is 99% on room air.  Physical examination demonstrates a hypotonic infant lying with arms and legs splayed outward on the examination table.  He has bilateral ptosis and poor suck.  Gag reflex is intact. The patient is admitted to the intensive care unit for close observation.  On admission, his temperature is 36.8 C (98.2 F) , blood pressure is 88/45 mm Hg, pulse is 135/min, respirations are 28/min, and pulse oximetry is 99% on room air.  His examination is otherwise unchanged from initial evaluation.  Arterial blood gas on room air is as follows:
Which of the following is the most appropriate immediate step in management of this patient?

A) Endotracheal intubation and mechanical ventilation
B) Intravenous ampicillin and gentamicin
C) Intravenous botulism immune globulin
D) Intravenous pooled immune globulin
E) Oral pyridostigmine