Which of the following is not seen in a child with cystic fibrosis?

Correct Answer: B. Hyperkalemia

Cystic fibrosis (CF) is an autosomal recessive disorder affecting the CFTR chloride channel, leading to thick, viscous secretions in the lungs, pancreas, and sweat glands. The cardinal electrolyte abnormality in CF is hypokalemia (not hyperkalemia), which occurs due to chronic loss of potassium in sweat and through gastrointestinal losses from pancreatic insufficiency and malabsorption. Patients with CF typically present with contraction alkalosis—a metabolic alkalosis driven by volume depletion from sweat losses of chloride and sodium, combined with renal compensation that conserves hydrogen ions. The sweat chloride concentration in CF is characteristically ≥60 mEq/L (diagnostic threshold), and values of 70 mEq/L are pathognomonic. Elevated immunoreactive trypsinogen (IRT) on newborn screening is a hallmark finding due to pancreatic ductal obstruction and acinar atrophy. Hyperkalemia is not a feature of CF; instead, the chronic electrolyte derangement is hypokalemia with metabolic alkalosis. This question tests understanding of CF's pathophysiology and the specific electrolyte pattern that distinguishes it from other conditions.

Why the other options are wrong

A. Contraction alkalosis — This is a classic finding in CF. Chronic sweat losses of sodium and chloride lead to volume depletion and hypochloremic metabolic alkalosis. The kidneys retain hydrogen ions to maintain electroneutrality, perpetuating the alkalosis. Contraction alkalosis is one of the hallmark acid-base disturbances in CF and is frequently tested in NEET PG. C. Sweat chloride test chloride conc of 70mEq/L — Elevated sweat chloride (≥60 mEq/L) is the diagnostic gold standard for CF. A value of 70 mEq/L is well above the diagnostic threshold and is a defining feature of the disease. This finding is present in virtually all CF patients and is used for confirmatory diagnosis after positive IRT screening. D. Increase immunoreactive trypsinogen level — Elevated IRT is a sensitive newborn screening marker for CF. Pancreatic ductal obstruction by thick secretions leads to acinar atrophy and reduced trypsin secretion into the duodenum, causing IRT to accumulate in serum. This is the primary screening tool in India's newborn screening programs for CF.

High-Yield Facts

Mnemonics

CF Electrolyte Pattern: HypoK-Alk Hypokalemia + Alkalosis = CF's electrolyte signature. Remember: Sweat losses → Cl⁻ depletion → volume contraction → metabolic alkalosis + K⁺ wasting. CF Screening & Diagnosis: IRT-Sweat IRT (elevated on newborn screening) → Sweat chloride test (≥60 mEq/L confirms CF). Two-tier approach used in Indian newborn screening programs.

NBE Trap

NBE pairs CF with "hyperkalemia" to trap students who confuse CF's electrolyte losses with conditions causing potassium retention (e.g., renal failure, ACE inhibitor use). The correct pattern is hypokalemia with metabolic alkalosis.

Clinical Pearl

In Indian pediatric practice, CF is often diagnosed late due to limited newborn screening access outside major centers. A child presenting with recurrent respiratory infections, failure to thrive, and steatorrhea should raise suspicion for CF. The sweat chloride test remains the gold standard confirmatory test, and early recognition allows for pancreatic enzyme supplementation and airway clearance therapy to prevent complications.

_Reference: OP Ghai Pediatrics Ch. 10 (Respiratory Disorders); Harrison Ch. 256 (Cystic Fibrosis)_