Answer to hypotonia: a simple hemogram.

A 1-year-old boy with weight loss, decreased activity, and psychomotor regression is presented. He was subjected to an extremely detailed evaluation, including electroencephalography (EEG) and magnetic resonance imaging (MRI), until a simple hemogram in our center revealed that he had macrocytic anemia with megaloblastic changes in the bone marrow. His history revealed that he had been exclusively breast-fed by his vegetarian mother. Further investigations showed low serum vitamin B12 concentration, methylmalonic aciduria, and homocysteinemia, indicating that the macrocytic anemia was due to vitamin B12 deficiency. This boy represents a case of macrocytic anemia and hypotonia owing to vitamin B12 deficiency that developed because of exclusive breast-feeding by a vegetarian mother.