A 16-month-old female is brought to the pediatric outpatient clinic by her parents due to concerns regarding weight loss and “failure to thrive.” The mother reports that the child was born at term following an uncomplicated pregnancy and had been tracking along the 50th percentile for weight until approximately 9 months of age. Since then, she has progressively dropped percentiles. The parents are perplexed because the child has a voracious appetite and consumes large portions of table food, in addition to whole milk. They deny any history of vomiting, diarrhea, greasy stools, or fevers.

Remarkably, the parents describe the child as “the happiest baby ever,” noting that she is extremely energetic, constantly moving, and requires very little sleep compared to her siblings. She has met all developmental milestones on time, including walking and first words. Social history reveals a stable two-parent household with no financial barriers to food access. Family history is negative for celiac disease, cystic fibrosis, or inflammatory bowel disease.

The child appears emaciated with marked loss of subcutaneous fat (lipodystrophy), particularly in the face and buttocks, She appears hyper-alert and euphoric, and is actively exploring the exam room. Her temperature is 98.6^o F (37^o C), heart rate is 110/bpm, respirations are 24/min, and blood pressure is 90/55 mm Hg. Her weight is 7.8 kg (<3rd percentile), height is 82 cm (75th percentile), head circumference is 48 cm (50th percentile), and BMI is 11.6 kg/m² (<1st percentile).