Final answer:
The L5-S1 lateral spot view is the most diagnostic radiographic position for assessing spondylolisthesis at the L5 - S1 level, providing a detailed view of the vertebral displacement.
Step-by-step explanation:
To diagnose the extent of spondylolisthesis at the L5 - S1 level in a patient, the most diagnostic radiographic position would be the L5-S1 lateral spot view. This imaging position provides a clear view of the alignment of the vertebrae and can demonstrate the degree of anterior or posterior displacement of the L5 vertebra in relation to the sacrum (S1). It is specifically focused on the affected area, offering a detailed assessment to guide potential treatment plans.