Methylmalonic aciduria and homocystinuria, cblC type, is a defect in B12 metabolism. Patients may present with severe early onset disease that includes megaloblastic and macrocytic anemia and failure to thrive, microcephaly, lethargy and feeding difficulties. Neurologic symptoms may include hypotonia, seizures, developmental delay, ataxia and optic atrophy. Late onset cases may present with neurologic symptoms such as anorexia, irritability, fatigue, myelopathy or dementia. Other complications are mental retardation, microcephaly, retinal degeneration, pigmentary retinopathy, microangiopathy and skin lesions.