What is the most likely diagnosis for an 8-month-old infant with failure to thrive, respiratory symptoms, malabsorption indicated by greasy stools, and salty skin?

Cystic Fibrosis

This 8-month-old infant has cystic fibrosis, as evidenced by the classic triad of failure to thrive with steatorrhea (loose, foul-smelling, greasy stools indicating pancreatic insufficiency), recurrent respiratory infections with chronic pulmonary symptoms (noisy breathing, cough, fine crackles), and salty-tasting skin. 1

Clinical Reasoning

The presentation is pathognomonic for cystic fibrosis based on multiple converging features:

Gastrointestinal Manifestations

• Pancreatic insufficiency is present at diagnosis in over 80% of CF patients, manifesting as steatorrhea (loose, foul-smelling, greasy stools) due to fat malabsorption from absent pancreatic enzyme activity 2, 1
• The abdominal distention and tenderness reflect malabsorption and intestinal dysmotility commonly seen in CF 2
• Failure to thrive despite adequate oral intake is the most frequent symptom in CF infants, present in 69% of cases diagnosed by newborn screening 3
• Growth parameters at the 30th percentile with static weight and height over two months indicate significant growth failure 2

Respiratory Manifestations

• Acute viral respiratory infections commonly develop into lower respiratory tract infections in CF infants, with this patient having had three upper respiratory infections in four months 1
• The chronic cough, noisy breathing, and bilateral fine crackles indicate ongoing pulmonary involvement with thick mucus production and airway obstruction 2
• Purulent nasal drainage reflects chronic sinopulmonary disease characteristic of CF 2

Pathognomonic Sign

• Salty-tasting skin is diagnostic of CF, resulting from defective salt reabsorption in sweat glands due to CFTR dysfunction 2, 1
• This finding alone, combined with the clinical presentation, essentially confirms the diagnosis 2

Differential Diagnosis Exclusion

The combination of symptoms excludes other conditions:

• Cardio-facio-cutaneous syndrome presents with failure to thrive and feeding difficulties but lacks the respiratory symptoms, steatorrhea, and salty skin seen in this patient 2
• Acid sphingomyelinase deficiency can cause failure to thrive and hepatosplenomegaly but does not produce the characteristic pulmonary findings or salty skin of CF 2
• Asthma can cause recurrent wheezing and cough but does not cause steatorrhea, failure to thrive, salty skin, or the degree of chronic pulmonary findings with purulent drainage seen in CF 1

Immediate Next Steps

This infant requires immediate referral to a specialized CF center to prevent irreversible complications 1:

• Sweat chloride test (gold standard for diagnosis, with values >60 mmol/L diagnostic) 4, 5
• CFTR genetic testing to identify specific mutations 4, 6
• Fecal pancreatic elastase-1 measurement (expected <100 μg/g stool confirming pancreatic insufficiency) 2

Critical Management Priorities

Immediate treatment must include 1:

• Pancreatic enzyme replacement therapy to address malabsorption 2, 1
• Fat-soluble vitamin supplementation (A, D, E, K) due to malabsorption 2
• Aggressive pulmonary care to prevent chronic Pseudomonas aeruginosa infection, which leads to irreversible lung damage and significantly impacts survival 1
• High-calorie nutritional support to address failure to thrive 2

Common Pitfall

The most critical error would be delaying diagnosis and referral to a CF center. Most CF infants are symptomatic in the first six weeks of life, yet the disease is frequently not suspected despite pediatric monitoring 3. The combination of failure to thrive, steatorrhea, and respiratory symptoms should immediately trigger CF evaluation, as early intervention dramatically improves outcomes 1, 3.