**Core Concept**
Positive Trendelenburg's sign is a clinical indicator of hip abductor weakness or pelvic instability. It is characterized by the descent of the opposite (non-operated) hip when a patient stands on the operated leg, indicating a disruption in the normal biomechanics of the hip joint.

**Why the Correct Answer is Right**
The Trendelenburg's sign is a result of the imbalance of hip abductors, primarily the gluteus medius and gluteus minimus muscles. When these muscles are weakened or paralyzed, the pelvis on the opposite side drops due to the unopposed action of the hip adductor muscles, such as the tensor fasciae latae and the adductor magnus. This sign is often seen in patients with hip abductor weakness, pelvic instability, or after hip surgery.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Trendelenburg's sign is indeed seen in hip abductor weakness, which can occur due to various conditions such as gluteus medius paralysis, hip abductor tendonitis, or hip abductor muscle pathology.

**Option B:** This option is incorrect because Trendelenburg's sign is a positive sign indicating hip abductor weakness, whereas a negative Trendelenburg's sign would indicate normal hip abductor function.

**Option C:** This option is incorrect because Trendelenburg's sign is not typically seen in patients with hip flexor weakness, as hip flexor weakness would not cause a pelvic drop on the opposite side.

**Option D:** This option is incorrect because Trendelenburg's sign is not typically seen in patients with lumbar spine instability, as lumbar spine instability would not directly affect the hip abductors.

**Clinical Pearl / High-Yield Fact**
To remember the Trendelenburg's sign, recall that it is a sign of hip abductor weakness, which can be caused by various conditions such as gluteus medius paralysis, hip abductor tendonitis, or hip abductor muscle pathology. It is essential to differentiate Trendelenburg's sign from other signs of pelvic instability or hip pathology.

**Correct Answer:** D