Cystic fibrosis (CF) questions are the ultimate “read every word” vignettes: one clue points to the diagnosis, and the answer choices test whether you truly understand what CF is doing at the cellular level—and how that cascades into pulmonary infections, GI disease, infertility, and electrolyte abnormalities. Let’s break down a classic USMLE-style stem and then dissect every option like you’re reviewing a missed Q-bank question.
Clinical vignette (USMLE-style)
A 7-year-old boy is brought to clinic for chronic cough and recurrent respiratory infections. His mother reports bulky, foul-smelling stools and poor weight gain despite a good appetite. He has had multiple episodes of pneumonia, and sputum culture previously grew Pseudomonas aeruginosa. Physical exam shows nasal polyps and digital clubbing. Sweat chloride is elevated.
Question: Which of the following best explains the underlying defect?
Correct answer: Defective CFTR → decreased chloride secretion with increased sodium absorption (thick secretions)
The key mechanism
CF is caused by mutations in CFTR, an ATP-gated chloride channel on the apical surface of epithelial cells (lungs, pancreas, intestine, sweat ducts, vas deferens).
- Most common mutation: ΔF508 (deletion of phenylalanine at position 508)
- Core pathophysiology (high yield):
- Lungs: ↓ Cl⁻ secretion + ↑ Na⁺ absorption → water follows Na⁺ → dehydrated airway surface liquid → thick mucus, impaired mucociliary clearance → infection + inflammation → bronchiectasis
- Pancreas: mucus plugging → pancreatic insufficiency → fat malabsorption → steatorrhea + fat-soluble vitamin deficiencies
- Sweat glands: CFTR normally reabsorbs Cl⁻ (and indirectly Na⁺) in the duct → defective CFTR → high sweat chloride
- Reproductive: congenital bilateral absence of vas deferens → obstructive azoospermia (infertility)
Why Pseudomonas matters
CF airways are a perfect setup for chronic colonization:
- Early: Staphylococcus aureus, H influenzae
- Later/classic: Pseudomonas aeruginosa (often mucoid; biofilms)
High-yield “CF cluster” for Step exams
- Recurrent sinopulmonary infections
- Nasal polyps
- Bronchiectasis
- Pancreatic insufficiency → steatorrhea
- Meconium ileus (newborns)
- Male infertility
- Elevated sweat chloride
Why every distractor matters (systematic breakdown)
Below are common answer choices designed to lure you away from CFTR physiology. Learn what they actually describe so you can eliminate them fast.
Distractor 1: “Defect in ciliary dynein arms causing impaired mucociliary clearance”
What it’s describing: Primary ciliary dyskinesia (Kartagener syndrome)
Why it’s tempting: both have chronic respiratory infections.
Why it’s wrong here:
- Kartagener is a motile cilia problem → impaired clearance, but no pancreatic insufficiency and no elevated sweat chloride.
- Classic associations:
- Chronic sinusitis + bronchiectasis
- Situs inversus
- Infertility (immotile sperm / impaired fallopian tube cilia)
Quick rule:
- CF = thick mucus (ion transport) + GI issues
- PCD = broken cilia (motility) + situs inversus clue
Distractor 2: “Mutation in alpha-1 antitrypsin leading to panacinar emphysema”
What it’s describing: Alpha-1 antitrypsin deficiency
Why it’s tempting: obstructive lung disease + younger patients.
Why it’s wrong here:
- A1AT deficiency causes emphysema (airspace destruction), not thick mucus/bronchiectasis as the primary problem.
- Usually:
- Panacinar emphysema (lower lobes)
- Liver disease (cirrhosis, hepatocellular carcinoma)
- Not associated with high sweat chloride, steatorrhea, or Pseudomonas colonization as the “classic bug.”
High-yield pearl: Smoking accelerates emphysema in A1AT deficiency because it increases elastase burden + inactivates remaining A1AT.
Distractor 3: “Hyperplasia of bronchial smooth muscle with eosinophilic inflammation”
What it’s describing: Asthma
Why it’s tempting: obstructive disease + cough/wheeze.
Why it’s wrong here:
- Asthma is reversible airway hyperreactivity; CF is viscous secretions + infection.
- Asthma clues:
- Atopy, nighttime symptoms
- Eosinophils, Charcot-Leyden crystals, Curschmann spirals
- Response to bronchodilators/steroids
- CF clues in stem:
- Steatorrhea/poor growth
- Nasal polyps
- Pseudomonas
- High sweat chloride
Distractor 4: “Failure of neural crest migration leading to aganglionosis”
What it’s describing: Hirschsprung disease
Why it’s tempting: GI symptoms in kids.
Why it’s wrong here:
- Hirschsprung = failure to pass meconium, abdominal distension, explosive stool after rectal exam.
- Path: absence of Auerbach + Meissner plexuses in distal colon.
- CF causes meconium ileus (bowel obstruction due to thick meconium), but the vignette is later childhood with pulmonary infections + pancreatic insufficiency.
Fast discriminator:
- Meconium ileus → think CF
- Delayed meconium + tight rectum → think Hirschsprung
Distractor 5: “Autoimmune destruction of exocrine glands leading to dry eyes and mouth”
What it’s describing: Sjögren syndrome
Why it’s tempting: exocrine gland dysfunction parallels “pancreatic issues.”
Why it’s wrong here:
- Sjögren = xerophthalmia + xerostomia, anti-Ro/SSA and anti-La/SSB, parotid enlargement, increased lymphoma risk.
- CF pancreatic insufficiency is due to duct obstruction and fibrosis, not autoimmune gland destruction.
Distractor 6: “Defective chloride reabsorption in the thick ascending limb (NKCC2)”
What it’s describing: Bartter syndrome (loop-diuretic-like)
Why it’s tempting: chloride handling and salt/water physiology.
Why it’s wrong here:
- Bartter affects kidney salt reabsorption → hypokalemic metabolic alkalosis, hypercalciuria.
- CF’s hallmark electrolyte issue is high sweat chloride, not a TAL transporter defect.
Rapid comparison table (high yield)
| Condition | Core defect | Classic clue | Key “separating” feature |
|---|---|---|---|
| Cystic fibrosis | CFTR mutation (ΔF508 common) → thick secretions; high sweat Cl⁻ | Recurrent infections + steatorrhea | Pseudomonas, pancreatic insufficiency |
| Primary ciliary dyskinesia | Dynein arm defect → impaired ciliary motility | Chronic sinusitis + bronchiectasis | Situs inversus |
| Alpha-1 antitrypsin deficiency | ↓ A1AT → ↑ elastase | Early emphysema | Panacinar (lower lobes) + liver disease |
| Asthma | Type I hypersensitivity, eosinophils | Atopy, episodic wheeze | Reversible obstruction |
| Hirschsprung | Aganglionosis (neural crest) | No meconium in newborn | Explosive stool on rectal exam |
USMLE “micro + management” add-ons they love to test
Bugs in CF airways
- Staph aureus and H influenzae early
- Pseudomonas aeruginosa later (mucoid biofilms)
- Burkholderia cepacia (worse prognosis; can spread between CF patients)
Complications worth memorizing
- Bronchiectasis (chronic cough + hemoptysis)
- Pancreatic insufficiency → fat malabsorption
- Fat-soluble vitamin (A, D, E, K) deficiencies
- CF-related diabetes (pancreatic damage)
- Rectal prolapse (kids; from bulky stools)
- Infertility in males (absent vas deferens)
Treatment hooks (Step 2 flavor)
- Airway clearance: chest physiotherapy
- Mucus thinning: inhaled hypertonic saline, dornase alfa
- Antibiotics targeting Pseudomonas when indicated (often inhaled tobramycin/aztreonam in chronic management; IV regimens for exacerbations)
- CFTR modulators (mutation-specific; commonly tested conceptually)
Take-home: how to nail CF vignettes fast
If the stem gives you sinopulmonary infections + pancreatic insufficiency + elevated sweat chloride, don’t overthink it: the unifying diagnosis is CF, and the unifying mechanism is defective CFTR causing thick, dehydrated secretions in the lungs and high chloride in sweat. Then use that mechanism to crush distractors—most wrong answers describe other obstructive diseases, but they won’t explain steatorrhea + Pseudomonas + nasal polyps + sweat chloride in one package.