This patient most likely has sacroiliitis, or inflammation of the sacroiliac (SI) joints. SI joints are true synovial joints between the sacrum and ilium of the pelvis. The SI joint may be involved as part of a systemic inflammatory syndrome such as spondyloarthritis, particularly ankylosing spondylitis, but may also be involved as an isolated musculoskeletal condition. Biomechanical factors that predispose to SI joint injury include repetitive torsional forces or unidirectional pelvic shear forces, as might occur with stepping off of a curb. Patients with leg length discrepancies or those with other conditions that may alter pelvic mechanics, such as pregnancy, scoliosis, or lumbar fixation, may also be at increased risk. The diagnosis of sacroiliitis is supported by the posterior location of this patient's hip pain and a positive FABER test, in which the hip is Flexed, ABducted, and Externally Rotated and gentle downward pressure is applied to the knee. This test has a high specificity for sacroiliitis and a somewhat lower sensitivity. Therapy for sacroiliitis is similar to that for other joint pain, including rest, anti-inflammatory medications, and possibly physical therapy. A number of additional treatments, including glucocorticoid injections, are used in patients who do not respond to conservative therapy.

Hip joint osteoarthritis causes pain directly in the hip joint that frequently radiates to the groin. Range of motion of the hip is usually limited and reproduces the pain. This patient's pain does not radiate, and his range of motion is normal, making this a less likely diagnosis.

Piriformis syndrome results from compression or limitation of the sciatic nerve by the piriformis muscle. It typically causes symptoms similar to sciatic nerve compression in the lumbosacral spine, with pain, tingling, and numbness that radiate into the leg, findings that are not present in this patient.

Trochanteric bursitis classically causes lateral hip pain over the greater trochanteric bursa, which is located over the greater trochanter lateral to the hip joint. Pain associated with trochanteric bursitis may radiate to the buttock or knee and is often worse when lying on the affected side. It does not affect range of motion. Trochanteric bursitis can be differentiated from hip joint pain based on its characteristic location relative to the pain associated with sacroiliitis.