Which test would be positive if the hip dislocates while the hips and knees are flexed at 90 degrees?

The Barlow test is designed to assess hip instability in infants, particularly the presence of developmental dysplasia of the hip (DDH). When hips and knees are flexed at 90 degrees, the Barlow maneuver involves applying a posterior pressure to the femur while the hip is held in abduction. A positive Barlow test occurs when the hip dislocates when this pressure is applied, which is indicative of hip instability.

In contrast, the Ortolani test also evaluates hip stability but serves specifically to reduce a dislocated hip rather than to cause dislocation, which is a key distinction between the two tests. The Trendelenburg test assesses gluteus medius strength and pelvic stability when standing on one leg, and the Thomas test checks for hip flexor tightness, not dislocation. Therefore, the Barlow test is the most appropriate choice in the context of detecting hip dislocation while the hips and knees are flexed at 90 degrees.