A 12-year-old boy with cystic fibrosis (CF) due to homozygous F508del CFTR mutations presents for routine pulmonary assessment. Spirometry shows FEV1 of 65% predicted with a flow-volume loop pattern marked **A** in the diagram. His sweat chloride is 98 mEq/L (normal

Question

A 12-year-old boy with cystic fibrosis (CF) due to homozygous F508del CFTR mutations presents for routine pulmonary assessment. Spirometry shows FEV1 of 65% predicted with a flow-volume loop pattern marked **A** in the diagram. His sweat chloride is 98 mEq/L (normal <30). He has been on standard CF therapy (airway clearance, inhaled tobramycin, dornase alfa) for 2 years with an annual FEV1 decline of approximately 2% per year. His parents ask about newer treatment options. Which of the following best describes the therapeutic mechanism of elexacaftor/tezacaftor/ivacaftor (TRIKAFTA) that would improve the obstructive pattern seen in structure **A**?

Accepted Answer

B. Corrects misfolded CFTR protein and facilitates its trafficking to the apical membrane, while potentiating channel opening to restore chloride and bicarbonate transport. ## Why option 1 is correct

The clinical anchor is that CFTR modulator therapy—specifically the combination of correctors (elexacaftor, tezacaftor) and potentiator (ivacaftor)—addresses the fundamental defect in CF: misfolded and mislocalized CFTR protein. The correctors improve protein folding and trafficking to the apical membrane, while the potentiator (ivacaftor) opens the channel pore, restoring chloride and bicarbonate transport. This reverses the viscous mucus production caused by impaired ion transport, thereby reducing airway obstruction and improving the FEV1 obstructive flow-volume loop pattern shown in structure **A**. Pivotal trials demonstrated FEV1 improvements of 10–14 percentage points and dramatic reductions in sweat chloride within weeks of starting TRIKAFTA (CFF Patient Registry 2023; Nelson Textbook of Pediatrics 22e).

## Why each distractor is wrong

- **Option 2 (Neutrophil elastase inhibition)**: While neutrophilic inflammation is a key driver of CF lung disease, elastase inhibitors are not the mechanism of CFTR modulators. Modulators work upstream by restoring CFTR function, not by targeting inflammation directly.
- **Option 3 (Aquaporin-3 activation)**: Increasing airway surface liquid is a mechanism of some CF therapies (e.g., hypertonic saline, amiloride), but CFTR modulators work by restoring the defective ion channel itself, not by activating alternative water channels.
- **Option 4 (Antimicrobial peptide upregulation)**: While chronic Pseudomonas infection is a hallmark of CF, CFTR modulators do not directly kill bacteria or upregulate antimicrobial defenses. Their benefit on infection rates is secondary to improved airway clearance and reduced inflammation from restored CFTR function.

**High-Yield:** CFTR modulators (correctors + potentiators) restore the defective chloride/bicarbonate channel at the apical membrane, reversing the pathophysiology of CF—not treating infection or inflammation directly.

[cite: CFF Patient Registry 2023; Nelson Textbook of Pediatrics 22e; Elexacaftor/Tezacaftor/Ivacaftor mechanism in CF]

Answer

A. Directly degrades biofilm-forming Pseudomonas aeruginosa by upregulating antimicrobial peptide production

Answer

C. Enhances mucociliary clearance by increasing airway surface liquid volume through aquaporin-3 activation

Answer

D. Inhibits neutrophil elastase to reduce airway inflammation and prevent further bronchiectasis progression

Explanation

Why option 1 is correct

Why each distractor is wrong

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