This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Orthopaedics
Least helpful for the diagnosis of spondylolisthesis - 
Select and explain. 
A. AP x-ray of spine, B. Lateral x-ray of spine, C. MRI, D. CT
A: AP x-ray of spine
Answer: A: AP x-ray of spine 
Explanation: All the given radiological investigations are used for spondylolisthesis. However, AP x-ray of spine is least helpful as in low grades of spondylolisthesis it may be normal.
On the other hand lateral x-ray, CT and MRI (sagittal view) diagnose even the mildest degree of spondylolisthesis.
It is worth noting that spondylolysis (not spondylolisthesis) may be missed on lateral x-ray and MRI. CT scanning is more sensitive in diagnosing spondylolysis.