Cystic Fibrosis — SAQ

Stem

A candidate is asked to manage a classic presentation of Cystic Fibrosis in an exam setting. Use precise definitions, scores, doses, and decision thresholds.

Core knowledge (model answer backbone)

Cystic fibrosis (CF) is an autosomal recessive multi-system disorder caused by loss-of-function mutations in the CFTR gene (chromosome 7q31.2; F508del accounts for ~70% of disease alleles). CFTR encodes a cAMP-regulated epithelial chloride/bicarbonate channel; its dysfunction produces dehydrated, viscous secretions across lungs, pancreas, gut, liver, sweat glands and reproductive tract. Pulmonary disease — chronic endobronchial infection (Staphylococcus aureus, Pseudomonas aeruginosa, Burkholderia cepacia complex) with bronchiectasis and respiratory failure — is the dominant cause of morbidity and mortality. Diagnosis rests on a positive newborn screen (immunoreactive trypsinogen), a sweat chloride over 60 mmol/L and confirmatory CFTR genotyping. Care has been transformed by CFTR modulator therapy — ivacaftor (gating mutations), lumacaftor/tezacaftor-ivacaftor and, above all, the triple combination elexacaftor-tezacaftor-ivacaftor (Trikafta/Kaftrio), effective in ~90% of patients — which has lifted median survival beyond 50 years. Comprehensive care also includes airway clearance, mucolytics (dornase alfa, hypertonic saline), inhaled antibiotics, macrolide immunomodulation, pancrea

Red flags

• Acute pulmonary exacerbation — increased cough, sputum volume or purulence, dyspnoea, fatigue, weight loss, FEV1 fall over 10 percent, fever, new crackles — needs prompt IV dual antibiotics per sputum culture, intensified airway clearance and nutritional support
• Massive haemoptysis (over 240 mL in 24 hours or recurrent significant bleeding) — urgent bronchial artery embolisation, IV antibiotics, protect airway, transfuse as needed
• Pneumothorax in CF — large or symptomatic — small-bore chest drain with suction, pleurodesis or surgical pleurectomy for recurrence; affects up to 3 to 4 percent lifetime
• Distal intestinal obstruction syndrome (DIOS) — right iliac fossa pain, distension, palpable mass, vomiting — oral or rectal Gastrografin, rehydration; surgery if perforation or refractory

High-yield structure examiners expect

Cover: Overview & Definition, Classification, Epidemiology & Risk Factors, Pathophysiology, Clinical Presentation, Differential Diagnosis.

Key doses / thresholds (from topic teaching)

• sweat chloride over 60 mmol/L** plus CFTR genotyping, after a p
• Greater than 60 mmol/L = diagnostic (in replicate)", "30–59
• concentrations above 60 mmol/L are diagnostic
• max 6 g/day) + tobramycin 5–7 mg/kg IV OD (
• max 6 g/day) + tobramycin, or
• max 18 g/day) + tobramycin, for 14 days

Questions

a) Define the condition and give the most important classification or severity framework used in exams. (3 marks)

• Clear one-line definition matching standard teaching.
• Named classification / stages / types with discriminating features.
• One sentence on why classification changes management.

b) Outline pathophysiology in a mechanism chain that explains the main clinical features. (3 marks)

• Initiating insult → intermediate pathway → end-organ effect.
• Link at least two symptoms/signs to mechanism.
• Mention one complication pathway (e.g. shock, perforation, herniation, arrhythmia).

c) List discriminating clinical features and bedside assessment. (3 marks)

• Classic presentation plus one atypical group (elderly, pregnancy, child, immunocompromised).
• Named signs/manoeuvres if relevant.
• What must never be missed on exam/bedside (pregnancy test, airway, glucose, etc.).

d) Investigations with thresholds and one named score if applicable. (3 marks)

• First-line tests and what positive findings mean.
• Gold-standard or definitive investigation when needed.
• Score components reproduced exactly if a named score is standard for this topic.

e) Immediate resuscitation and definitive management with doses where standard. (3 marks)

• ABC / time-critical steps first.
• First-line drug(s) with agent + dose + route (or procedure steps).
• Escalation triggers (theatre, ICU, thrombolysis window, antidote, etc.).
• Disposition and safety-netting.

Marking tips

Full marks require specificity (numbers, names, doses) not generic "give antibiotics/fluids." Regional practice (ICMR / NICE / AHA) may be cited as alternative where relevant.