Cystic Fibrosis
The most likely diagnosis is cystic fibrosis (Option D). This 8-month-old infant presents with the classic triad of CF: failure to thrive with steatorrhea (loose, foul-smelling, greasy stools indicating pancreatic insufficiency), recurrent respiratory infections with chronic pulmonary symptoms, and the pathognomonic finding of salty-tasting skin.
Clinical Presentation Strongly Supports Cystic Fibrosis
The constellation of symptoms in this infant is diagnostic of CF. The key features include:
Failure to thrive with steatorrhea: The infant's weight and height have plateaued at the 30th percentile over two months despite adequate intake, accompanied by loose, foul-smelling, and greasy stools 1. This presentation reflects pancreatic insufficiency, which is present at diagnosis in over 80% of CF patients and causes fat malabsorption with characteristic steatorrhea and growth failure 1.
Salty skin: The mother's observation that the infant's skin tastes salty when kissed is a highly specific clinical finding for CF, reflecting the elevated chloride concentration in sweat that results from CFTR dysfunction 2, 3, 4.
Recurrent respiratory infections with chronic symptoms: Three URIs in four months with persistent noisy breathing, cough, fine bilateral crackles, and purulent nasal drainage indicate chronic pulmonary involvement 1. Acute viral respiratory infections commonly develop into lower respiratory tract infections in CF infants, and chronic cough typically becomes evident around 10.5 months of age 1.
Why Other Diagnoses Are Excluded
Acute bronchitis (Option A), croup (Option C), and pertussis (Option E) are excluded because these are acute, self-limited respiratory illnesses that do not explain the chronic multisystem presentation with gastrointestinal symptoms, failure to thrive, and salty skin 1.
Asthma (Option B) is excluded because while it can cause recurrent wheezing and cough in infants, it does not cause steatorrhea, failure to thrive, salty skin, or the degree of chronic pulmonary findings with purulent drainage seen in this patient 1. The guideline notes that episodic wheezing and cough in infants under 5 years can be caused by viral infections or asthma, but CF must be excluded in the differential diagnosis of children with these symptoms 1.
Diagnostic Confirmation Required
The diagnosis must be confirmed with sweat chloride testing, which remains the gold standard for CF diagnosis 2, 3, 4. An elevated sweat chloride concentration (>60 mmol/L) combined with this clinical presentation confirms the diagnosis 4.
Genetic testing for CFTR mutations should be performed to identify the specific mutations, which guides prognosis and increasingly allows for mutation-specific therapies 1, 3.
Fecal chymotrypsin or fecal elastase testing can confirm pancreatic insufficiency, which is present in 85% of symptomatic CF infants 5.
Critical Management Implications
Immediate referral to a specialized CF center is essential to prevent irreversible complications 3. The infant requires:
- Pancreatic enzyme replacement therapy with high-fat diet to address malabsorption and promote catch-up growth 1
- Aggressive pulmonary care to prevent chronic Pseudomonas aeruginosa infection, which leads to irreversible lung damage and significantly impacts survival (53% survival to age 16 with chronic P. aeruginosa versus 84% without) 1
- Fat-soluble vitamin supplementation to prevent deficiencies from malabsorption 1
Common Pitfall to Avoid
CF is frequently underdiagnosed in symptomatic infants because failure to thrive is often attributed to other causes such as gastroesophageal reflux, recurrent pneumonia, or chronic bronchitis 1, 5. Most CF infants diagnosed by newborn screening are already symptomatic in the first six weeks of life, with failure to thrive being the most frequent symptom—44% weigh the same as or less than their birth weight 5. In any infant with growth failure despite adequate caloric intake, especially when accompanied by steatorrhea or recurrent respiratory infections, CF must be excluded by sweat testing 5, 4.