What is the most likely diagnosis in a 10‑month‑old infant with irritability during feeding, recurrent vomiting, weight below the 3rd percentile, length and head circumference at the 25th percentile, and two healthy older siblings?

Organic Failure to Thrive

The most likely diagnosis is organic failure to thrive (Option A), as the infant presents with specific gastrointestinal symptoms (irritability during feeding and recurrent vomiting) combined with disproportionate growth failure where weight is severely affected (below 3rd percentile) while length and head circumference remain at the 25th percentile—a pattern characteristic of acute malnutrition from an identifiable organic pathology rather than psychosocial deprivation. 1

Key Diagnostic Features Supporting Organic Etiology

Disproportionate Growth Pattern:

• Weight below the 3rd percentile with height and head circumference at the 25th percentile indicates acute malnutrition affecting weight first, which is the critical distinguishing feature of organic failure to thrive 1
• This pattern demonstrates that calories are being lost or not absorbed despite intake, rather than simple inadequate intake seen in non-organic causes 1

Specific Gastrointestinal Symptoms:

• The combination of irritability during feeding with recurrent vomiting points to specific organic pathology such as gastroesophageal reflux disease (GERD), swallowing dysfunction, or other gastrointestinal pathology 1, 2
• These are not simply behavioral feeding issues but suggest a physiologic problem preventing adequate nutrition 1

Why Other Options Are Less Likely

Non-organic Failure to Thrive (Option B) is unlikely because:

• Non-organic FTT typically presents with decreased intake without specific GI symptoms like vomiting 3
• The presence of two healthy older siblings makes psychosocial deprivation or inadequate parenting practices unlikely 1
• Non-organic causes would not typically produce the specific symptom constellation of feeding irritability plus vomiting 1, 3

Chromosomal Abnormality (Option C) is unlikely because:

• Normal birth history with two healthy siblings makes chromosomal abnormalities unlikely 1
• Chromosomal abnormalities typically affect all growth parameters proportionally (weight, length, and head circumference), not just weight 1

Normal Variation in Growth (Option D) is excluded because:

• Weight below the 3rd percentile (2.3rd percentile threshold) indicates potential adverse health conditions requiring evaluation 1, 4
• The presence of symptoms (vomiting, feeding irritability) definitively rules out normal variation 1

Immediate Clinical Approach

Initial Evaluation:

• Video swallow study to assess for swallowing dysfunction and aspiration risk 1
• Extended 24-hour esophageal pH monitoring, barium swallow, and gastric scintiscan to evaluate for GERD 1
• Complete blood count, comprehensive metabolic panel, and thyroid function tests to exclude systemic disease 1, 2

Specialized Testing if Initial Workup Negative:

• Sweat chloride test for cystic fibrosis (sensitivity 90%, specificity 99%), as pancreatic insufficiency from CF commonly presents with normal birth weight followed by growth failure in early infancy with weight faltering before height 1
• Fecal elastase to assess pancreatic function if pancreatic insufficiency suspected 1

Monitoring:

• Serial weight measurements every 2-4 weeks initially to track response to interventions, with expected weight gain of approximately 300-400g per month 1
• Immediate gastroenterology referral given the vomiting and feeding irritability 1

Common Pitfalls to Avoid

• Do not dismiss symptoms as behavioral when specific GI symptoms are present—the combination of vomiting with feeding irritability demands organic investigation 1, 3
• Do not delay evaluation waiting for further weight loss—intervention should occur when downward trajectory is clear 1
• Avoid extensive laboratory screening in truly non-organic cases (which this is not), as routine evaluation yields positive findings in <1.4% of asymptomatic cases 5