c - Idiopathic pulmonary fibrosis (IPF) falls within the class of restrictive lung diseases, which is defined as "loss of lung tissue or limited lung expansion due to decreased compliance of the lung or muscle weakness". Forced vital capacity (FVC) and vital capacity are generally less than 80% of normal values. Also included in this class are such conditions as pectus excavatum, myasthenia gravis, and space occupying lesions.