A 58-year-old woman complained to her physician of increasing dyspnea and cough over the past few days. The woman had a 22-pack-year history of cigarette smoking and had been suffering from chronic obstructive pulmonary disease for 5 years. She was currently on inhaled ipratropium and albuterol. Her physician decided to add a drug that acts by selectively inhibiting phosphodiesterase 4 in bronchial muscle. Which of the following drugs was most likely prescribed?