Final answer:
The drug most likely to have caused the theophylline toxicity is Cimetidine.
Step-by-step explanation:
The drug most likely to have caused the theophylline toxicity in this case is Cimetidine. Cimetidine is a medication used to treat acid peptic disease, and it is known to inhibit the enzyme responsible for the metabolism of theophylline in the liver. This inhibition can result in an increased plasma concentration of theophylline, leading to toxicity.