**Core Concept:**
Peripherally acting muscle relaxants are medications that primarily affect skeletal muscle relaxation at the neuromuscular junction, excluding the central nervous system (CNS). The correct answer refers to a muscle relaxant that has additional effects beyond peripheral neuromuscular blockade.

**Why the Correct Answer is Right:**
The correct answer, option C (Diazepam), is a benzodiazepine that not only acts peripherally but also crosses the blood-brain barrier and exerts its effects in the central nervous system. Diazepam is a sedative-hypnotic with anxiolytic, muscle relaxant, and anticonvulsant properties, acting on the gamma-aminobutyric acid (GABA) receptors.

**Why Each Wrong Option is Incorrect:**

A. Option A (Pancuronium) is a non-depolarizing neuromuscular blocking agent, acting primarily at the neuromuscular junction and not crossing the blood-brain barrier.
B. Option B (Orphenadrine) is a centrally acting muscle relaxant, acting on both peripheral and central muscarinic receptors, and not specifically targeting skeletal muscle relaxation.
C. Option D (Diazepam) is a benzodiazepine with central actions, including muscle relaxation, as explained above.

**Clinical Pearl / High-Yield Fact:**
**Correct Answer: C (Diazepam)**

Diazepam, although a peripheral muscle relaxant, is more than just a peripheral agent because it has significant central effects as well. This dual action makes it useful in conditions like myotonia where both peripheral and central mechanisms are involved. However, in purely peripheral muscle relaxants, you would choose options A (pancuronium), B (orphenadrine), or D (methocarbamol).