A 30-year-old woman comes to the physician because of a 2-day history of abdominal pain. She has a history of recurrent upper respiratory tract infections, sinusitis, and pancreatitis. She has thick n

A 30-year-old woman comes to the physician because of a 2-day history of abdominal pain. She has a history of recurrent upper respiratory tract infections, sinusitis, and pancreatitis. She has thick nasal secretions. She says that her sweat is salty and crystallizes on her skin. Her vital signs are within normal limits. Physical examination shows epigastric tenderness. Genetic testing for the 36 most common mutations shows a detectable mutation (G551D) in one allele of the CFTR gene. Which of the following best explains this patient's clinical phenotype?