HI, My baby is 15 months old. Her weight is 9.4 kg, head circumference is 46 cm. She appropriately gained her milestones till 12 months of age, she was able to sit without support, was able to crawl, stand with support, climb one stair and come down, was able to move walker but then she had cough without fever which continued for about 15 to 20 days after then she undergone regression in her milestones, with 2 months she is now unable to sit without support, cannot crawl. On standing she is standing on her tip toes. On stretching legs there is involuntary movement of foot, she is tightening her legs upon standing or sitting, she is keeping her left hand in fist position. MRI brain with contrast is normal, EMG, NCV is normal, Serum Ca, vit D levels, plasma amino acids, urine organic acid, serum lactate, vit B12 levels are normal. What could be the reason? Serum CPK is slightly raised.