Severe Methylmalonic Acidemia Precipitated by Dietary B12 Deficiency in Vegan Toddler.

A previously healthy 19-month-old female presented after her family found her upon awakening to be sleepy, uninterested in eating, and lethargic. On arrival, blood gas showed profound metabolic acidosis with normal lactate that did not improve with normalization of glucose and sodium. Her acidosis continued to worsen, so a broad workup was initiated, considering ingestions, DKA, and inborn errors of metabolism. In collecting further history, the patient's nutrition history included a combination of breastfeeding and a vegan diet followed by her family. Her B12 level returned undetectably low. She improved dramatically over the next several hours after receiving an injection of cyanocobalamin. Ultimately, she was confirmed to have methylmalonic acidemia secondary to severe Vitamin B12 deficiency.