A 1-year-old female infant is hospitalized for pneumonia. Bacterial cultures of the sputum have grown Pseudomonas aeruginosa. She has had two prior hospitalizations for severe respiratory infections. Her mother has noted that when she kisses her child, the child tastes "salty." The child has had weight loss that the mother attributes to frequent vomiting and diarrhea with bulky, foul-smelling fatty stools. The child is small for her age. Which of the following critical proteins is altered in this condition?

Cystic fibrosis, Most common lethal genetic disease in Caucasian populations. Defect in the cystic fibrosis transmembrane conductance regulator (CFTR) protein Deletion of phenylalanine in position 508 (DF508 mutation). Affected patients often have multiple pulmonary infections and pancreatic insufficiency with steatorrhea and failure to thrive. Death is often due to respiratory failure secondary to repeated pulmonary infections, facilitated by the buildup of thick, tenacious mucus in the airways. Increased concentration of chloride in sweat and tears is characteristic