**Core Concept**
The abduction contracture at the hip joint refers to a condition where there is a limitation in the range of motion of the hip joint, specifically in the abduction movement. This is often seen in patients with hip flexion contractures or in individuals who have been bedridden for a prolonged period.

**Why the Correct Answer is Right**
The Thomas test is a clinical examination technique used to assess the abduction contracture at the hip joint. It involves the patient lying on their back with their knees bent and feet flexed towards their buttocks. The examiner then checks if the patient's unaffected leg can be lifted towards the ceiling without the affected leg being lifted off the table. If the affected leg cannot be lifted, it indicates an abduction contracture. This test takes advantage of the fact that the iliopsoas muscle, which is responsible for hip flexion, is also responsible for limiting hip abduction.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the Thomas test is not a test for hip rotation or external rotation contracture.
**Option B:** This option is incorrect because the Ober test is used to assess the length of the tensor fasciae latae and iliotibial tract, not abduction contracture.
**Option C:** This option is incorrect because the flexion contracture test is not a specific test for abduction contracture at the hip joint.

**Clinical Pearl / High-Yield Fact**
The Thomas test can be used to differentiate between true hip flexion contractures and pseudocontractures, which are often seen in patients with hip joint pain or arthritis.

**Correct Answer: C. The Ober test is used to assess the length of the tensor fasciae latae and iliotibial tract, not abduction contracture at the hip joint.