This study is a 12-week randomized-controlled trial of memantine hydrochloride (Namenda) for the treatment for social impairment in adolescents with autism spectrum disorder (ASD). The investigators will also conduct pre- and post-treatment neuroimaging (fMRI and HMRS) to assess neural functional deficits in adolescents with ASD compared to healthy volunteer adolescents and to assess any effects of memantine therapy on neural function in adolescents with ASD. The investigators hypothesize that short-term memantine monotherapy will be safe, well-tolerated, and effective in improving the core symptoms of autism spectrum disorder in adolescents with ASD. Additionally, the investigators hypothesize that following memantine therapy, ASD subjects will exhibit a decrease in glutamate (Glu) concentration in the anterior-cingulate cortex (ACC) and a change towards normalization in altered functional connectivity of the ACC and medial temporal lobes, consistent with improvement in social impairments in ASD. The investigators hypothesize that compared to healthy volunteer subjects, ASD subjects will significantly differ on neuroimaging measures at baseline but that following memantine therapy, the difference between ASD and healthy volunteer neuroimaging data will decrease.