The Incorrect Statement is D: Chronic Airway Colonization with Pneumococcus
Statement D is incorrect because cystic fibrosis patients characteristically develop chronic colonization with Pseudomonas aeruginosa and Staphylococcus aureus, not pneumococcus (Streptococcus pneumoniae).
Analysis of Each Statement
Statement A: Presence of 2 Mutations (1 from Each Parent) - CORRECT
Cystic fibrosis is an autosomal recessive disorder requiring biallelic CFTR mutations (one from each parent) for disease manifestation 1, 2. This is the fundamental genetic basis of CF.
Statement B: Elevated Sweat Chloride Concentration - CORRECT
Elevated sweat chloride is the diagnostic gold standard for CF. Levels >60 mmol/L are diagnostic 1. The CF Foundation guidelines reference sweat chloride testing as reflecting the basic defect in CF, with treated patients showing significant decreases in sweat chloride concentration 3.
Statement C: Cough, Dyspnea, and Wheezing - CORRECT
Respiratory symptoms are nearly universal in CF patients. Historical data shows that cough and sputum production occur in virtually all CF patients, with the first respiratory symptoms typically developing before age 5 in the vast majority of cases 4. These symptoms reflect the chronic lung disease that characterizes CF.
Statement D: Chronic Airway Colonization with Pneumococcus - INCORRECT
This is the key error. The characteristic pathogens in CF are:
- Staphylococcus aureus (common early colonizer) 2
- Pseudomonas aeruginosa (hallmark pathogen in CF) 3, 4, 5
- Haemophilus influenzae 4
- Nontuberculous mycobacteria (e.g., Mycobacterium abscessus) 2
The CF Foundation guidelines specifically address chronic management of P. aeruginosa colonization with inhaled tobramycin and azithromycin 3, emphasizing that persistent P. aeruginosa in airway cultures is the expected pattern requiring targeted therapy. The acquisition of mucoid Pseudomonas strains signifies poor prognosis 4.
Pneumococcus (Streptococcus pneumoniae) is NOT a characteristic colonizing organism in CF airways. While CF patients may occasionally develop pneumococcal infections like any other individual, chronic colonization with pneumococcus is not a recognized feature of CF pulmonary disease.
Clinical Pitfall
A common error is assuming all respiratory pathogens affect CF patients similarly to immunocompromised hosts. However, CF creates a unique airway environment that selectively favors specific organisms—particularly Pseudomonas aeruginosa—due to the thick, dehydrated mucus and impaired mucociliary clearance. Recognition of P. aeruginosa colonization should prompt consideration of CF diagnosis in adolescents and young adults with chronic lung disease 5.
Answer: D is the incorrect statement.