You are answering a USMLE-style medical question. Question: A 16-year-old boy presents to the emergency department with shortness of breath after prolonged exposure to cold air during a recent hike with his friends. He informs the physician that he is asthmatic, but does not use inhalers regularly because he does not like using medications. He is a non-smoker and occasionally drinks alcohol. On physical examination, the temperature is 37.0°C (98.6°F), the pulse is 120/min, the blood pressure is 114/76 mm Hg, and the respiratory rate is 32/min. Auscultation of the chest reveals bilateral wheezing. The physician asks the nurse to administer nebulized albuterol; however, the boy declines nebulized albuterol because of a history of palpitations that he experienced previously. The physician then prescribes nebulized ipratropium bromide, which results in significant clinical improvement. Which of the following second messenger systems is affected by the drug that improved the boy's symptoms? Options: A) Cyclic guanosine monophosphate (cGMP) system B) Arachidonic acid system C) Phosphoinositol system D) Tyrosine kinase system Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.