**Core Concept**
The question is testing the understanding of Wallerian degeneration, a process that occurs after a nerve injury. Wallerian degeneration is the breakdown of the distal segment of a severed nerve, leading to the disruption of nerve function.

**Why the Correct Answer is Right**
The first change observed in the distal part of a cut nerve is the disruption of the axonal transport mechanism, which leads to the accumulation of organelles and proteins within the axon. This is followed by the breakdown of the myelin sheath and the axon itself, resulting in the loss of nerve function. The process of Wallerian degeneration is mediated by various enzymes, including acid phosphatase, which breaks down the myelin sheath.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the first change observed in the distal part of a cut nerve is not the loss of myelin sheath, but rather the disruption of axonal transport.
* **Option B:** This option is incorrect because the first change observed in the distal part of a cut nerve is not the accumulation of calcium ions, but rather the disruption of axonal transport and the breakdown of the myelin sheath.
* **Option C:** This option is incorrect because the first change observed in the distal part of a cut nerve is not the activation of the Schwann cells, but rather the disruption of axonal transport and the breakdown of the myelin sheath.

**Clinical Pearl / High-Yield Fact**
Wallerian degeneration is a critical process that occurs after nerve injury, and understanding its mechanisms is essential for the diagnosis and treatment of nerve injuries.

**Correct Answer:** C. Depolymerization of neurofilaments