**Core Concept**
March is a gait disturbance characterized by a short, shuffling step, often seen in patients with neurological disorders such as Parkinson's disease, cerebellar ataxia, or peripheral neuropathy. It results from the combination of weakened or slowed muscle contractions and impaired balance control.

**Why the Correct Answer is Right**
The commonest site of march is the lower limb, particularly the distal parts, due to the involvement of the corticospinal tract and the cerebellar pathways that regulate motor coordination and balance. In conditions like Parkinson's disease, the nigrostriatal pathway is affected, leading to rigidity, bradykinesia, and impaired motor control, which manifests as a short, shuffling step. The distal lower limb, including the foot and ankle, is often the most affected area due to the loss of proprioception and balance control.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because march is not typically associated with the upper limb. While some neurological conditions can cause upper limb dysfunction, march is a gait disturbance that primarily affects the lower limb.
**Option B:** This option is incorrect because march is not a characteristic of the trunk or torso. The trunk is not typically involved in the gait disturbance associated with march.
**Option C:** This option is incorrect because while the proximal lower limb (thigh) can be involved in some neurological conditions, the distal lower limb (foot and ankle) is more commonly affected in march.

**Clinical Pearl / High-Yield Fact**
The "heel-to-toe" walking pattern is a classic feature of march, where the patient takes small steps and places the heel of one foot directly in front of the toes of the other foot, indicating impaired balance control and motor coordination.

**Correct Answer: C. Lower limb.