Glycogen storage disease type III

 A 6-month male infant is referred to the pediatrician's office for the evaluation of an inborn error of metabolism. Medical history is relevant for low-weight and growth below the third percentile since birth. On physical examination, the baby is quivering and looks irritable, on abdominal palpation he has marked hepatomegaly, his liver edge is 4 cm below the right costal margin. His temperature is 36.7°C (98°F), pulse is 110/min, respirations are 30/min, blood pressure is 120/80 mmHg. Blood glucose is 45 mg/dL (2.5 mmol/L). A liver biopsy shows abnormal glycogen deposits, which contain single glucose residue at branches.  A deficiency of which of the following enzyme activities is the most likely cause of the findings in this patient?