Failure to thrive (FTT) describes inadequate physical growth in infants and young children, defined as weight below the 3rd percentile for age and sex, weight-for-length below the 3rd percentile, weight deceleration crossing two or more major percentile lines on the growth chart, or weight <80% of ideal body weight for age. The pathophysiology centers on an imbalance between caloric intake and caloric requirements: either INADEQUATE INTAKE (the most common mechanism, accounting for ~80% of FTT cases), INADEQUATE ABSORPTION (malabsorption from GI disease), EXCESSIVE LOSSES (chronic vomiting, diarrhea, renal wasting), or INCREASED METABOLIC DEMAND (chronic infection, congenital heart disease, hyperthyroidism). Historically, FTT was classified as ORGANIC (identifiable medical cause) vs. NON-ORGANIC (psychosocial or environmental etiology). Current understanding recognizes that most FTT involves a combination of both factors, and the term 'growth faltering' is now preferred. ORGANIC causes include: GI (celiac disease, cow's milk protein allergy, GERD, pyloric stenosis, inflammatory bowel disease, cystic fibrosis), endocrine (hypothyroidism, growth hormone deficiency, adrenal insufficiency), cardiac (congenital heart disease with increased metabolic demands), renal (chronic kidney disease, renal tubular acidosis), neurological (cerebral palsy with oromotor dysfunction,...