Step 1 topics → Respiratory → Cystic Fibrosis

Cystic Fibrosis

USMLE Step 1 trap: Misidentifies the primary ion affected by CFTR dysfunction as sodium rather than chloride. CFTR is a chloride channel; its dysfunction prevents Cl⁻ secretion into the airway lumen, causing secondary Na⁺ and water reabsorption that dehydrates mucus.

Cystic fibrosis is the highest-yield autosomal recessive disease on USMLE Step 1, and it's tested from every angle — genetics, pathophysiology, diagnosis, and management. The core defect is a dysfunctional CFTR chloride channel, but the downstream consequences span the lungs, pancreas, sweat glands, liver, and reproductive tract. You need to understand not just what breaks, but why each organ behaves differently depending on whether CFTR normally secretes or reabsorbs chloride in that tissue.

Step 1 tests CF at multiple levels of complexity. At the recall level, you need to know the mutation class of ΔF508, the sweat chloride cutoff, and which organisms colonize CF lungs at which ages. At the application level, you'll get a vignette — a newborn with meconium ileus, a teenager with recurrent sinopulmonary infections and failure to thrive, or a young man with infertility — and you have to work through the diagnostic algorithm or explain the mechanism. Passage-based questions often give you a scenario involving a new CFTR modulator and ask you to predict which mutation class benefits from it.

The tricky part is that students conflate two different roles CFTR plays: it secretes Cl⁻ into the airway lumen but reabsorbs Cl⁻ from sweat ducts. Getting those directions backward generates the two most common CF misconceptions — predicting low sweat chloride and misidentifying which ion is primarily affected in airway epithelium. On top of that, the ΔF508 mutation class and modulator eligibility are consistently misunderstood. Nail these distinctions and CF becomes one of the most reliable point-scorers on the exam.

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Common misconceptions

Common mistake
Wrong: CFTR dysfunction causes sodium to accumulate inside airway epithelial cells.
Right: CFTR is a chloride channel; its dysfunction prevents Cl⁻ secretion into the airway lumen, causing secondary Na⁺ and water reabsorption that dehydrates mucus.
CFTR is a chloride channel — full stop. When it fails, Cl⁻ cannot be secreted into the airway lumen. Because the lumen becomes electrically imbalanced, Na⁺ is reabsorbed via ENaC to compensate, dragging water with it. So Na⁺ movement is a secondary consequence, not the primary defect — if you anchor on 'sodium problem' you'll get mechanism questions wrong and mispredict treatment targets.
Common mistake
Wrong: Sweat chloride is low in CF because CFTR fails to secrete chloride into sweat.
Right: Sweat chloride is elevated (>60 mEq/L) in CF because CFTR normally reabsorbs Cl⁻ from sweat ducts; without functional CFTR, Cl⁻ remains in sweat.
In sweat ducts, CFTR's job is reabsorption — it pulls Cl⁻ back out of forming sweat before it reaches the skin. Without functional CFTR, that reabsorption fails and Cl⁻ stays in the sweat, making it saltier than normal. This is why sweat chloride is elevated (>60 mEq/L) in CF, and why the classic description is 'salty-tasting sweat.' Confusing secretion with reabsorption here is one of the most common CF errors on USMLE Step 1.
Common mistake
Wrong: ΔF508 is a class I mutation causing no protein production.
Right: ΔF508 is a class II mutation causing misfolded CFTR protein that is degraded in the ER before reaching the cell surface.
Class I mutations produce no functional protein at all (nonsense/frameshift). ΔF508 is class II: the protein is made but misfolded, so it gets flagged and degraded in the ER before it ever reaches the apical membrane. This distinction matters clinically — class II proteins can potentially be rescued by correctors (like elexacaftor/tezacaftor) that stabilize the fold, whereas class I proteins have nothing to rescue.
Common mistake
Wrong: CFTR modulators like ivacaftor work for all CF patients including those with ΔF508 homozygous mutations as monotherapy.
Right: Ivacaftor monotherapy works only for gating mutations (e.g., G551D); ΔF508 patients require combination therapy (elexacaftor/tezacaftor/ivacaftor) to correct misfolding and potentiate the channel.
Ivacaftor alone is a potentiator — it holds open CFTR channels that make it to the cell surface but open too infrequently (gating mutations like G551D). ΔF508 channels are degraded in the ER and never reach the surface, so there's nothing for ivacaftor to potentiate as monotherapy. You need a corrector (elexacaftor + tezacaftor) to stabilize and traffic the misfolded protein first, then ivacaftor to potentiate it once it's there. Applying ivacaftor monotherapy to ΔF508 patients on the exam will cost you points.
Common mistake
Gap: Fails to recognize meconium ileus as a near-pathognomonic neonatal presentation of CF
Meconium ileus at birth is virtually pathognomonic for CF and should prompt immediate sweat chloride testing and CFTR genotyping.
Meconium ileus — bowel obstruction in the first 24-48 hours of life due to inspissated meconium in the terminal ileum — occurs in about 15% of CF newborns but is so strongly associated that it should immediately make you think CF. No other condition causes meconium ileus with the same frequency. Any vignette with a newborn who fails to pass meconium and has abdominal distension should prompt you to order sweat chloride testing and CFTR genotyping, even before pulmonary symptoms appear.
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What the exam tests

  1. Know CF inheritance (autosomal recessive, chromosome 7), the ΔF508 mutation class (class II — misfolding/trafficking defect), and how different mutation classes inform which modulator therapy applies.
  2. Understand the direction of CFTR's action in each tissue: it secretes Cl⁻ into airway lumen (so dysfunction dehydrates mucus) but reabsorbs Cl⁻ from sweat ducts (so dysfunction raises sweat chloride) — these are opposite effects from the same channel defect.
  3. Know the CF diagnostic algorithm: newborn screen (IRT) → sweat chloride test (>60 mEq/L is diagnostic) → CFTR genotyping if equivocal, and recognize meconium ileus at birth as a near-pathognomonic trigger for immediate workup.
  4. Recognize the multi-organ picture: recurrent Pseudomonas/Staph pulmonary infections leading to bronchiectasis, pancreatic exocrine insufficiency with fat-soluble vitamin deficiency (A/D/E/K), meconium ileus in neonates, and CBAVD causing male infertility.
  5. Distinguish CFTR modulator mechanisms: ivacaftor is a potentiator (opens gating-mutant channels, works for G551D as monotherapy) while ΔF508 requires a corrector + potentiator combination (elexacaftor/tezacaftor/ivacaftor) because the misfolded protein must first be rescued from ER degradation before it can be potentiated.

Can you avoid these mistakes?

A newborn presents with abdominal distension and failure to pass meconium in the first 48 hours. X-ray shows a 'soap bubble' appearance in the right lower quadrant. What is the most likely diagnosis, and what is the next best diagnostic step?
A CF patient is homozygous for ΔF508. His pulmonologist wants to start CFTR modulator therapy. Why is ivacaftor monotherapy insufficient, and what combination regimen would you use instead?
Explain why sweat chloride is elevated in CF even though CFTR dysfunction in the airways leads to reduced luminal chloride. What does this tell you about CFTR's direction of transport in each tissue?
A 22-year-old man is found to have congenital bilateral absence of the vas deferens (CBAVD) on infertility workup. He has no pulmonary or GI symptoms. His sweat chloride is 45 mEq/L (borderline). What should you do next, and what is the likely underlying diagnosis?

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