methylmalonic acidemia due to transcobalamin receptor defect (MONDO_0013341) can be described as follows. Methylmalonic aciduria due to transcobalamin receptor defect is a rare metabolite absorption and transport disorder characterized by a moderate increase of methylmalonic acid (MMA) in the blood and urine due to decreased cellular uptake of cobalamin resulting from decreased transcobalamin receptor function. Patients are usually asymptomatic however, screening reveals increased C3-acylcarnitine and MMA in plasma. Serum homocysteine levels may vary from normal to moderately elevated and retinal vascular occlusive disease, resulting in severe visual loss, has been reported. Also known as: CD320 methylmalonic acidemia, methylmalonic acidemia caused by mutation in CD320, methylmalonic acidemia, TCb1R type, methylmalonic acidemia, TCbIR type, methylmalonic aciduria due to transcobalamin receptor defect.