Which feature is characteristic of cystic fibrosis?

• A. Low sweat chloride
• B. Elevated sweat chloride
• C. Absence of exocrine gland involvement
• D. Decreased mucus production

Answer: (B)

Elevated sweat chloride is characteristic because defective CFTR transport in cystic fibrosis impairs chloride reabsorption in the sweat ducts. Normally, chloride is reabsorbed in the duct, and sodium follows, producing sweat with low salt content. When CFTR is nonfunctional, chloride reabsorption drops, sodium follows less efficiently, and the sweat becomes unusually salty. This is the basis of the diagnostic sweat test, where high chloride values (typically over about 60 mmol/L) strongly suggest CF.

Other statements don’t fit CF: low sweat chloride would not occur since CF causes salty sweat; CF involves exocrine glands and leads to thick, dehydrated secretions, not absence of involvement or decreased mucus; mucus production is increased in viscosity, not reduced, contributing to airway obstruction.