**Core Concept**
Transtentorial herniation is a life-threatening condition where brain tissue is forced through a narrow opening in the tentorium cerebelli, which separates the cerebrum from the cerebellum. This displacement leads to increased intracranial pressure, compromising blood flow and function of various brain structures.

**Why the Correct Answer is Right**
In transtentorial herniation, the uncus of the temporal lobe is typically the part of the brain that is displaced from the supratentorial to the infratentorial compartment. As it herniates, it compresses the oculomotor nerve (cranial nerve III) against the posterior clinoid process, leading to ipsilateral oculomotor paresis (Option D). The patient may also exhibit Cheyne-Stokes respiration due to the involvement of the medulla oblongata and the brainstem (Option A). Decerebration can occur as the herniation progresses, manifesting as a posturing response characterized by extension of the arms and legs (Option B). However, ipsilateral hemiparesis is not a direct consequence of transtentorial herniation, as the motor pathways are located in the cerebral hemispheres, which are not directly affected by the herniation.

**Why Each Wrong Option is Incorrect**
**Option A:** Cheyne-Stokes respiration is a common finding in transtentorial herniation, resulting from the compression of the medulla oblongata.
**Option B:** Decerebration is a possible consequence of transtentorial herniation, especially as the herniation progresses and the brainstem is compromised.
**Option D:** Ipsilateral oculomotor paresis is a direct result of the compression of the oculomotor nerve by the herniating uncus.

**Clinical Pearl / High-Yield Fact**
Transtentorial herniation can be remembered as the "Dandy-Walker syndrome of the brain," where the normal brain is displaced into an abnormal position, leading to catastrophic consequences.

**✓ Correct Answer: C. Ipsilateral hemiparesis**