Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Pulmonary alveolar proteinosis acquired ?
    ### Response:
    Acquired pulmonary alveolar proteinosis (PAP) is a rare, acquired lung disorder characterized by the accumulation of grainy material consisting mostly of protein and fat (lipoproteinaceous material) in the air sacs of the lungs (alveoli). Most cases affect adults between the ages of 20-50.
    