A 7-month-old child presented with neuro-developmental delay, loss of motor skills, increased stale reaction to noise, feeding difficulties & failure to thrive. On examination, macrocephaly was noted along with myoclonus and spasticity. There was no evidence of any corneal clouding or angiokeratomatas. On investigating fuher, CT scan reveals no hepatosplenomegaly and no cardiac involvement. Fundoscopy reveals - Which enzyme deficiency is responsible for above symptoms?