Final answer:
Gaenslen's test is used to differentiate between a disc lesion and SI joint involvement, where pain at the SI joint during the test indicates SI joint problems.
Step-by-step explanation:
The test used to differentiate between a disc lesion and sacroiliac (SI) joint involvement is d) Gaenslen's test. This test involves the patient lying on their back at the edge of an examination table, bringing the knee on the unaffected side to the chest to maximize flexion, and extending the affected leg back over the edge of the table. If pain is elicited at the sacroiliac joint, it indicates SI joint involvement. On the other hand, a Straight Leg Raise (SLR) test is commonly used to assess for disc herniation, whereas Kemp's test is used for facet joint pain, and Braggard's test is an extension of the SLR test to further test for sciatic nerve irritation due to disc herniation.