A 54 year old man is admitted to the hospital with cough and dyspnea, which over a period of years has led to marked respiratory embarrassment and cyanosis. Chest x-ray film of the lungs showed bilateral lower lobe ground-glass infiltrates. Wedge biopsy of the lung demonstrates air spaces filled with macrophages containing lipid, periodic acid-Schiff (PAS)-positive granules, and lamellar bodies. There is an accompanying interstitial pneumonitis, hyperplasia of the septal lining epithelial cells, and desquamation of epithelial cells into alveolar. The lamellar bodies within the macrophages are composed of which of the following?