Also known as: CF, cystic fibrosis, CFTR, mucoviscidosis
Overview
Autosomal recessive multisystem disorder caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene on chromosome 7, leading to dysfunctional chloride and bicarbonate transport, thick viscous secretions, and progressive damage to lungs, pancreas, intestines, hepatobiliary tract, and reproductive system.
Epidemiology
Most common life-shortening autosomal recessive disease in white populations (~1:2500 live births); ~30,000 affected in the US. Median predicted survival in patients born now exceeds 50 years thanks to CFTR modulators and aggressive care.
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Question 1PulmonaryMedium
A 25-year-old woman with cystic fibrosis who is homozygous for the F508del mutation has an FEV1 of 65% predicted and frequent pulmonary exacerbations despite airway clearance and inhaled antibiotics. Which of the following CFTR modulator regimens is most appropriate?
ATezacaftor/ivacaftor dual therapy
BLumacaftor/ivacaftor dual therapy
CElexacaftor/tezacaftor/ivacaftor
DIvacaftor potentiator monotherapy
Reveal answer & full explanation
Correct answer: C — Elexacaftor/tezacaftor/ivacaftor
ATezacaftor/ivacaftor dual therapy
BLumacaftor/ivacaftor dual therapy
CElexacaftor/tezacaftor/ivacaftor✓
DIvacaftor potentiator monotherapy
Why Elexacaftor/tezacaftor/ivacaftor is correct
Elexacaftor/tezacaftor/ivacaftor (ETI) is the most effective CFTR modulator and is the standard of care for patients with at least one F508del allele, including F508del homozygotes
It pairs two correctors (elexacaftor, tezacaftor) with a potentiator (ivacaftor), improving CFTR protein folding, trafficking, and channel gating
In trials it raised FEV1 by roughly 14 percentage points, cut pulmonary exacerbations substantially, and lowered sweat chloride while improving BMI
It is appropriate for the great majority of people with CF, making it first-line modulator therapy here
Why the others are wrong
Tezacaftor/ivacaftor dual therapy — a dual corrector/potentiator approved for F508del homozygotes but less effective than triple therapy; right class, lesser drug (right-diagnosis-wrong-step)
Lumacaftor/ivacaftor dual therapy — an earlier dual modulator for F508del homozygotes with smaller FEV1 gains and more drug interactions and chest tightness than ETI (premature closure on an older agent)
Ivacaftor potentiator monotherapy — a potentiator effective only for gating mutations such as G551D, not for F508del homozygotes, who need a corrector (buzzword-matching "ivacaftor")
Question 2PulmonaryMedium
A 19-year-old woman with cystic fibrosis presents with worsening dyspnea, increased purulent sputum, and a 4 kg weight loss over 3 weeks. Sputum culture grows Pseudomonas aeruginosa. FEV1 has declined 18% from baseline. Which intravenous antibiotic regimen is most appropriate?
AIV tobramycin plus piperacillin-tazobactam
BIV ciprofloxacin plus vancomycin
CIV vancomycin plus cefepime
DIV azithromycin plus ceftriaxone
Reveal answer & full explanation
Correct answer: A — IV tobramycin plus piperacillin-tazobactam
AIV tobramycin plus piperacillin-tazobactam✓
BIV ciprofloxacin plus vancomycin
CIV vancomycin plus cefepime
DIV azithromycin plus ceftriaxone
Why IV tobramycin plus piperacillin-tazobactam is correct
Acute pulmonary exacerbations of cystic fibrosis (CF) due to Pseudomonas are treated with two anti-pseudomonal agents to broaden coverage and limit resistance
An aminoglycoside (tobramycin) plus an antipseudomonal beta-lactam (piperacillin-tazobactam, ceftazidime, or meropenem) is the standard combination
Why the others are wrong
B) IV ciprofloxacin plus vancomycin — vancomycin targets MRSA rather than Pseudomonas; oral ciprofloxacin monotherapy would be reserved for mild exacerbations and is inadequate for this degree of FEV1 decline and weight loss
C) IV vancomycin plus cefepime — vancomycin targets MRSA rather than Pseudomonas
D) IV azithromycin plus ceftriaxone — azithromycin provides only anti-inflammatory benefit in CF and is not exacerbation therapy; ceftriaxone lacks reliable anti-pseudomonal activity
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Celiac disease — Failure to thrive, malabsorption with normal sweat chloride; positive tTG IgA, biopsy
Diagnostic workup
Diagnostic criteria
CF Foundation 2017: clinical features OR positive newborn screen OR sibling history PLUS evidence of CFTR dysfunction — sweat chloride ≥60 mmol/L (two tests) OR two disease-causing CFTR mutations OR abnormal nasal potential difference.
Labs
Sweat chloride test (pilocarpine iontophoresis) — diagnostic gold standard: ≥60 mmol/L on two occasions confirms CF; 30-59 intermediate; <30 normal
CFTR gene panel (>2000 mutations identified)
Newborn screening: immunoreactive trypsinogen (IRT) + CFTR mutation panel (now universal in US — diagnosis in infancy in most cases)
Pulmonary exacerbations: IV antibiotics tailored to sputum culture — typically antipseudomonal beta-lactam (cefepime, ceftazidime, piperacillin-tazobactam) + aminoglycoside (tobramycin) for 10-14 days
Chronic azithromycin 250-500 mg 3×/week — anti-inflammatory benefit even without infection
CFRD: insulin (oral hypoglycemics generally inadequate); maintain high-calorie diet
Osteopenia, infertility (especially male CBAVD), nephrolithiasis
Allergic bronchopulmonary aspergillosis (ABPA)
PANCE pearls
Sweat chloride ≥60 mmol/L on two occasions is diagnostic; with newborn screening, most US patients are diagnosed in infancy.
Elexacaftor/tezacaftor/ivacaftor (Trikafta) has dramatically changed CF prognosis — eligible for ~90% of patients with ≥1 F508del or responsive mutation.
Chronic Pseudomonas colonization marks a shift toward inhaled antibiotic suppression therapy (tobramycin or aztreonam cycles).
Burkholderia cepacia complex isolation is often a transplant contraindication and a poor prognostic marker.
CFRD requires insulin (NOT oral agents) and a high-calorie, unrestricted diet — distinct from type 1 or 2 diabetes management.
References
CF Foundation — Cystic Fibrosis Foundation Patient Registry Annual Data Report (2023)
Trikafta Trial — Elexacaftor-Tezacaftor-Ivacaftor for CF with a Single F508del Allele (Middleton et al., NEJM 2019)
CFF Diagnosis — Diagnosis of Cystic Fibrosis: Consensus Guidelines from the Cystic Fibrosis Foundation (Farrell et al., J Pediatr 2017)
CFF Care Guidelines — Cystic Fibrosis Pulmonary Guidelines: Chronic Medications (Mogayzel et al., Am J Respir Crit Care Med 2013)
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