Everything You Need to Know About Cystic fibrosis for Step 1

Cystic fibrosis (CF) is one of those “Step 1 forever” diseases: it links genetics → ion transport → thick secretions → chronic lung infections + pancreatic dysfunction. If you can tell a clean story from CFTR mutation to bronchiectasis and pancreatic insufficiency, you’ll pick up easy points across pulm, GI, peds, and micro.

Where CF Fits: Obstructive Lung Disease (and why)

CF is classically tested as an obstructive lung disease because it causes:

• Airway plugging with thick mucus
• Air trapping and wheezing
• Progressive bronchiectasis (permanent dilation of bronchi from chronic infection/inflammation)

Pulmonary function tests (PFTs): obstructive pattern

• ↓ FEV1
• ↓ FEV1/FVC
• Often ↑ RV (air trapping)

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First Aid cross-reference: Respiratory—Obstructive Lung Diseases; Cystic Fibrosis; Bronchiectasis

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Definition (Step-ready)

Cystic fibrosis is an autosomal recessive disease caused by mutation in the CFTR gene (chromosome 7) leading to defective epithelial chloride transport, which results in dehydrated, thick secretions in the lungs, pancreas, and other organs.

Classic Step phrase: “Defective CFTR → thick mucus → recurrent lung infections + pancreatic insufficiency.”

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First Aid cross-reference: Genetics—Autosomal Recessive Disorders; Respiratory—Cystic Fibrosis; GI—Pancreatic insufficiency

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Pathophysiology: From Ion Channels to Infection (the high-yield chain)

1) The core defect: CFTR dysfunction

CFTR is an epithelial chloride channel regulated by cAMP.

• Most common mutation: ΔF508
  • Misfolding → protein degradation → CFTR doesn’t reach cell surface
• Net effect in many epithelia:
  • ↓ Cl⁻ secretion
  • ↑ Na⁺ absorption (via ENaC) in airway epithelium
  • Water follows sodium → dehydrated airway surface liquid → thick mucus

2) Why lungs get wrecked

Thick mucus → impaired mucociliary clearance → bacterial colonization → neutrophilic inflammation → airway wall damage → bronchiectasis.

Key micro associations (very high-yield):

• Staphylococcus aureus (early; common in kids)
• Pseudomonas aeruginosa (later; chronic colonization)
  • Often mucoid strains due to alginate biofilm
• Burkholderia cepacia (can cause severe, rapid decline; transplant complications)
• Aspergillus can show up (e.g., ABPA overlap in some patients)

3) Why pancreas and GI are involved

Thick secretions obstruct pancreatic ducts → autodigestion/inflammation → exocrine pancreatic insufficiency:

• Fat malabsorption → steatorrhea, weight loss/failure to thrive
• Fat-soluble vitamin deficiencies: A, D, E, K
• Over time: pancreatic damage can lead to CF-related diabetes (loss of islet function)

4) Sweat glands: the board-style trick

In sweat ducts, CFTR normally reabsorbs Cl⁻, and Na⁺ follows.

• CFTR defect → ↓ Cl⁻ reabsorption → ↑ sweat chloride
• This is why babies can taste “salty.”

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First Aid cross-reference: Respiratory—Cystic Fibrosis; Endocrine—Diabetes; Biochem—Vitamins; Micro—Pseudomonas

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Clinical Presentation (what they’ll describe in vignettes)

Pulmonary

• Chronic cough with sputum
• Recurrent pneumonias
• Wheezing, dyspnea
• Bronchiectasis (chronic productive cough, hemoptysis in advanced cases)
• Nasal polyps and chronic sinusitis (common clue)

GI / Nutrition

• Failure to thrive in infants/children
• Bulky, greasy stools (steatorrhea)
• Distention, poor weight gain despite appetite
• Meconium ileus in newborns (intestinal obstruction)

Reproductive

• Male infertility due to congenital bilateral absence of the vas deferens (CBAVD) → obstructive azoospermia
• Females can have reduced fertility from thick cervical mucus (less emphasized but testable)

Other

• Digital clubbing (chronic hypoxemia)
• Episodes of dehydration/hypochloremic metabolic alkalosis in hot weather (salt loss)

High-yield “classic CF kid” vignette:

• Recurrent lung infections + greasy stools + poor growth + nasal polyps + salty skin

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First Aid cross-reference: Repro—Male infertility; GI—Meconium ileus; Respiratory—Bronchiectasis; ENT—Nasal polyps

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Diagnosis (Step 1 loves the logic)

Newborn screening (common test stem)

Often detects elevated immunoreactive trypsinogen (IRT) (reflects pancreatic injury), then confirmatory testing follows.

Confirmatory test: Sweat chloride

Pilocarpine iontophoresis sweat test:

• Elevated sweat chloride supports CF.

Genetics

• CFTR mutation testing is used for confirmation and family counseling.
• Also relevant when evaluating:
  • Male infertility due to CBAVD
  • Siblings/relatives

Supporting findings

• Obstructive PFTs
• Imaging: hyperinflation; bronchiectasis on CT

Quick table: Diagnosis at a glance

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First Aid cross-reference: Respiratory—Cystic Fibrosis (sweat test), Micro—Pseudomonas; Pediatrics—Newborn screening

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Treatment (mechanism-based, board-friendly)

CF management is multidisciplinary. For Step, organize it by airway clearance + infection control + nutrition + CFTR modulators.

1) Airway clearance (treat the mucus)

• Chest physiotherapy (postural drainage, percussion, oscillatory devices)
• Inhaled hypertonic saline: draws water into airway lumen to hydrate mucus
• Dornase alfa (DNase): breaks down DNA from neutrophils → reduces mucus viscosity
• Bronchodilators may be used symptomatically in some patients

2) Antibiotics (treat the bugs)

• Treat acute exacerbations and suppress chronic colonization.
• Inhaled tobramycin is a classic association for chronic Pseudomonas.
• Culture-guided regimens are typical in real life; Step questions often just want you to recognize Pseudomonas and treat appropriately.

3) Reduce inflammation (select patients)

• Azithromycin is used in many CF patients for anti-inflammatory and anti-biofilm benefits (also antibacterial activity).

4) Nutrition + pancreatic support (easy points)

• Pancreatic enzyme replacement (lipase)
• Fat-soluble vitamin supplementation: A, D, E, K
• High-calorie diet; monitor growth in kids

5) CFTR modulators (Step 1 increasingly tests “precision therapy”)

These target the underlying CFTR defect. You don’t need every brand name, but you should know the concept:

• Potentiators (improve channel opening at cell surface)
• Correctors (improve folding/trafficking to cell surface)

High-yield link: ΔF508 is a trafficking/misfolding problem → correctors are especially relevant.

6) Advanced disease

• Oxygen for hypoxemia
• Management of complications (hemoptysis, pneumothorax)
• Lung transplant in end-stage disease (note: CF can recur in other organs, but transplanted lungs have functional CFTR from donor tissue)

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First Aid cross-reference: Pharm—Respiratory drugs; Micro—Pseudomonas coverage; GI—Vitamin deficiencies; Respiratory—Cystic Fibrosis treatment overview

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High-Yield Associations & “Buzz Phrases”

Micro “most likely organism”

• CF + recurrent lung infections: think S. aureus early, Pseudomonas later
• Pseudomonas in CF often described as mucoid, biofilm, alginate

GI

• Meconium ileus in newborn
• Pancreatic insufficiency → steatorrhea + ADEK deficiency

Reproductive

• CBAVD → male infertility

Endocrine

• CF-related diabetes (pancreatic damage)

Path

• Bronchiectasis: chronic necrotizing infections → permanent airway dilation
  • Symptoms: chronic productive cough, recurrent infections, hemoptysis

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Classic USMLE-Style Mini-Stems (train your reflexes)

• “Newborn with intestinal obstruction + failure to pass meconium” → CF → meconium ileus
• “Teen with chronic cough, recurrent pneumonias, salty skin, nasal polyps” → CF
• “CF patient with worsening cough, green sputum; culture shows oxidase+ non-lactose fermenter” → Pseudomonas
• “Male infertility with absent vas deferens” → consider CFTR mutation (even without full CF phenotype)

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Rapid Review Checklist (what you should be able to say in 15 seconds)

• AR mutation in CFTR (Chr 7), commonly ΔF508 (misfolding/trafficking defect)
• Thick mucus → recurrent infections + bronchiectasis
• S. aureus early, Pseudomonas later (mucoid biofilm)
• Pancreatic insufficiency → steatorrhea + ADEK deficiency; newborn meconium ileus
• High sweat chloride (sweat test)
• Tx: airway clearance (hypertonic saline, dornase alfa), antibiotics (e.g., inhaled tobramycin for Pseudomonas), pancreatic enzymes, vitamins, CFTR modulators