A 6-month-old male presented to the emergency department during the COVID-19 pandemic with 3-day history of fever, nasal congestion, cough, and decreased oral intake. He was not able to sit with support or roll over, and his growth parameters were severely reduced (<1st percentile). He tested negative for COVID-19, and had an unrevealing newborn screen. Severe failure to thrive (FTT) had been evident since his well-child visit at age 2 months. […] Because FTT was evident at 2 months in this child, this case represented a lost opportunity for early diagnosis and management of PMM2-CDG during the COVID-19 pandemic.