A 55-year-old man describes dyspnea on exertion, which has progressively gotten worse over the last 2 months. He also has developed a persistent dry cough and has lost weight. He does not report any travel, drug use, allergies or smoking. He used to work as a postal worker but retire because of his recent dyspnea. On examination, he appears anorexic and fatigued. He has bluish facial features, and clubbing is present. Auscultation reveals dry crackles. Laboratory work is normal, and the chest x-ray shows a diffuse bilateral reticulonodular pattern. A biopsy is done and reveals "idiopathic pulmonary fibrosis." What is the treatment of choice for this patient?