An individual with a protein deficiency typically experiences increased susceptibility to illnesses, muscle and joint pain, and difficulty concentrating. why does a protein deficiency cause such diverse symptoms? a. because immune, nervous, and muscle system cells require proteins for structure and regulation b. because the only role of proteins is to provide the enzymes that all cells need to function c. because the membranes of immune, muscle, and nervous system cells consist entirely of proteins d. because proteins are the primary source of energy for many different cell types