Final answer:
The nerve root at risk during the complete reduction of an L5/S1 Isthmic spondylolisthesis is the L5 nerve root, due to its anatomical position relative to the involved vertebrae.
Step-by-step explanation:
The nerve root that is most at risk with complete reduction of an L5/S1 Isthmic spondylolisthesis is the L5 nerve root. Spondylolisthesis refers to the anterior displacement of a vertebra or the vertebral column in relation to the vertebrae below. In the case of L5/S1 slippage, the L5 vertebra slips over the S1 vertebra. When attempting a reduction, meaning repositioning the displaced vertebra back to its normal alignment, there is risk of compression or stretching of the adjacent nerve roots. The L5 nerve root passes over the dorsal part of the S1 vertebral body, making it particularly susceptible to injury during such a procedure.