16 year old girl consults her primary care doctor  for primary amenorrhea and absence of breast development. Clinical examination reveals a male morphotype, signs of virilization with hypertrophy of the clitoris. Pelvic ultrasound confirmed the presence of the uterus and ovaries. 17- hydroxyprogesterone levels were elevated and the diagnosis of congenital adrenal hyperplasia is confirmed

 What is the  most likely enzyme deficiency responsible for this condition?