Correct Answer: A. Cystic fibrosis
Cystic fibrosis (CF) is diagnosed by elevated sweat chloride levels (>60 mEq/L is diagnostic; 40–60 mEq/L is borderline). The sweat chloride test is the gold standard for CF diagnosis in India and globally. CF is an autosomal recessive disorder affecting the CFTR (cystic fibrosis transmembrane conductance regulator) protein, which normally reabsorbs chloride in sweat ducts. When CFTR is defective, chloride reabsorption fails, causing abnormally high chloride concentration in sweat. This test is non-invasive, highly sensitive, and specific. Indian pediatric guidelines (IAP) recommend sweat chloride testing for any child with recurrent respiratory infections, pancreatic insufficiency, meconium ileus, or failure to thrive. The test is performed using pilocarpine iontophoresis (sweat stimulation) followed by chloride measurement. CF presents with chronic suppurative airway disease, pancreatic insufficiency (leading to steatorrhea and malabsorption), and elevated immunoreactive trypsinogen (IRT) on newborn screening. Early diagnosis via sweat test allows timely intervention with airway clearance, pancreatic enzyme replacement, and nutritional support, significantly improving outcomes in Indian CF patients.
Why the other options are wrong
B. Osteogenesis imperfecta — Osteogenesis imperfecta is a collagen synthesis disorder (COL1A1/COL1A2 mutations) diagnosed by clinical features (blue sclerae, bone fragility, hearing loss) and genetic testing, NOT sweat chloride levels. This is a trap for students who confuse genetic disorders with metabolic/secretory defects. Sweat testing is irrelevant to bone collagen pathology. C. Phenylketonuria — Phenylketonuria (PKU) is diagnosed by elevated plasma phenylalanine (>20 mg/dL) on newborn screening (Guthrie test or tandem MS), not sweat chloride. PKU is a phenylalanine hydroxylase deficiency. NBE may pair this with CF because both are screened in newborns, but the diagnostic biomarker is completely different—amino acid, not electrolyte. D. Gaucher's disease — Gaucher's disease is a lysosomal storage disorder (β-glucosidase deficiency) diagnosed by elevated acid phosphatase, chitotriosidase, and bone marrow biopsy showing Gaucher cells, not sweat chloride. This is a metabolic storage disease unrelated to sweat gland function or CFTR. Sweat testing has no role in Gaucher's diagnosis.
High-Yield Facts
- Sweat chloride >60 mEq/L is diagnostic for cystic fibrosis; 40–60 mEq/L requires repeat testing or genetic confirmation.
- CFTR protein defect prevents chloride reabsorption in sweat ducts, causing elevated sweat electrolytes (sodium and chloride).
- Pilocarpine iontophoresis is the standard method to stimulate sweat collection for CF diagnosis in India.
- CF presents with chronic suppurative airway disease, pancreatic insufficiency, and meconium ileus in neonates.
- Newborn screening in India uses immunoreactive trypsinogen (IRT) as first-tier marker; sweat test confirms diagnosis.
- Pancreatic enzyme replacement and airway clearance are cornerstones of CF management in Indian pediatric practice.
Mnemonics
CF Diagnosis: SWEAT Sweat chloride >60 = diagnostic; Weak CFTR protein; Elevated electrolytes in sweat; Autosomal recessive; Test via pilocarpine iontophoresis. Use this to remember that sweat chloride is THE diagnostic test for CF. CF Triad: PAM Pancreatic insufficiency (steatorrhea); Airway disease (chronic suppurative); Meconium ileus (neonatal presentation). Helps recall CF's multi-system involvement when considering differential diagnosis.
NBE Trap
NBE pairs CF with other genetic/metabolic disorders (osteogenesis imperfecta, PKU, Gaucher's) to test whether students know the SPECIFIC diagnostic biomarker for each. The trap is confusing "genetic disorder" with "sweat chloride test"—many students may recognize CF as genetic but not recall that sweat chloride is its unique diagnostic hallmark.
Clinical Pearl
In Indian pediatric practice, a child presenting with recurrent lower respiratory tract infections, failure to thrive, and fatty stools should trigger immediate sweat chloride testing. Early diagnosis via sweat test (rather than waiting for advanced lung damage) allows timely pancreatic enzyme replacement and airway clearance, dramatically improving quality of life and survival in CF patients.
_Reference: OP Ghai Pediatrics Ch. 12 (Cystic Fibrosis); Harrison Ch. 256 (Cystic Fibrosis)_