**Question:** Trendelenberg's gait is due to the weakness of -

**Core Concept:** Trendelenberg's gait is a clinical sign characterized by a tripping or stumbling gait, observed in patients with weakness in the hip flexors or lumbar paraspinal muscles.

**Why the Correct Answer is Right:** Trendelenberg's gait results from the weakness of the hip flexors, specifically the rectus femoris muscle and its primary motor neuron, the ventral horn of the L2-L3 spinal cord segment. This weakness causes the affected individual to step out to the side while walking, leading to the characteristic gait.

**Why Each Wrong Option is Incorrect:**

A. The weakness of the hip extensors (gluteus medius and gluteus minimus) would lead to a wide-based gait, not Trendelenberg's gait.
B. Weakness of the lumbar paraspinal muscles (erector spinae) would cause a postural instability rather than a specific gait abnormality.
C. Weakness of the hip abductors (gluteus medius and gluteus minimus) would result in a wide-based gait, not Trendelenberg's gait.
D. Weakness of the knee extensors would cause a genu recurvatum (knee flexion) rather than the Trendelenberg's gait pattern.

**Clinical Pearl:** Trendelenberg's gait is an important clinical clue for diagnosing neurogenic causes of muscle weakness, such as spinal cord compression, transverse myelitis, or multiple sclerosis. Involvement of the ventral horn motor neurons in these conditions can lead to the observed gait pattern.

**Correct Answer:** D. Weakness of knee extensors (quadriceps) results in genu recurvatum (knee flexion) instead of Trendelenberg's gait pattern.