A restriction is described as P-A with S-I only; which listing is most likely?

• A. PI
• B. IN
• C. EX
• D. AS

Answer: (D)

The key idea is that these two-letter listings encode the directions in which a vertebral segment is restricted. The first letter represents a sagittal-plane direction (anterior or posterior), and the second letter represents a vertical or craniocaudal direction (superior or inferior). If the restriction is described as restricted only in posterior-to-anterior and in superior-to-inferior directions, the code that reflects those restriction directions is the combination of anterior and superior: AS. The other codes would indicate restrictions in different directional pairs (for example, posterior with inferior, or other combinations) that don’t match the described pattern. So the listing most likely is anterior–superior.