A 75-year-old male patient is a known case of a pulmonary disease and is being treated with inhalational coicosteroids and daily theophylline.The patient was admitted to a hospital for urinary retention few days back and catheterization was done. However, the condition got complicated with urinary tract infection and the patient was put on an antibiotic for the same. Now the patient is presenting with symptoms such as nausea, vomiting, abdominal pain, headache and a fine hand tremor. The patient also appears to have tachycardia. According to the doctor, the symptoms may be due to increased serum levels of the drugs taken by the patient. Which of the following drug may be responsible for the patient's condition?