Vitamin B12 deficiency in an infant secondary to nutritional deficiency and an inadequate maternal diet.

Vitamin B12 (cobalamin, Cbl) is an essential micronutrient for DNA synthesis and neurological development. Its deficiency in infants, although infrequent in developed countries, can cause megaloblastic anemia, psychomotor delay, and neurological damage that may become irreversible if not treated early. We present the case of a 16-month-old infant, breastfed with partial refusal of solid foods, whose mother followed a diet poor in animal products and had low levels of Cbl in breast milk. The patient consulted for fever and somnolence, with clinical findings of hypotonia, global developmental delay, and pancytopenia. Laboratory workup confirmed severe Cbl deficiency with hyperhomocysteinemia and elevated methylmalonic acid. Congenital and malabsorptive causes were excluded. Intramuscular Cbl therapy led to rapid hematological and neurological improvement, together with nutritional support adapted due to cow's milk protein allergy. This case highlights the importance of early detection and adequate supplementation in at-risk mothers and infants.