A drug that relaxes the smooth muscles of the bronchial passages is which?

• A. Acetaminophen
• B. Lithium
• C. Phenytoin
• D. Theophylline

Answer: (D)

Relaxing the smooth muscle of the bronchial passages is the action of a bronchodilator, specifically theophylline in this set. Theophylline works mainly by inhibiting phosphodiesterase enzymes in airway smooth muscle, which raises intracellular cAMP. Higher cAMP causes the muscle to relax, widening the airways and easing airflow. It can also block adenosine receptors, which can further promote bronchodilation. Because of a narrow therapeutic index, dosing and interactions (like with certain antibiotics or caffeine) are important.

Acetaminophen is an analgesic/antipyretic with no bronchodilating effect. Lithium is a mood stabilizer. Phenytoin is an anticonvulsant. None of these relax bronchial smooth muscle, so theophylline is the correct choice.