**Core Concept**
Transtentorial uncal herniation is a type of brain herniation that occurs when the uncus of the temporal lobe is displaced across the tentorial notch, compressing the adjacent brain structures. This can lead to increased intracranial pressure, brainstem compression, and potentially life-threatening complications.

**Why the Correct Answer is Right**
Transtentorial uncal herniation typically causes compression of the third cranial nerve (oculomotor nerve), which can result in ipsilateral pupillary dilation and contralateral ptosis. It can also cause compression of the cerebral peduncle, leading to ipsilateral hemiparesis. Additionally, it can cause compression of the posterior cerebral artery, leading to ipsilateral occipital lobe ischemia.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not a typical consequence of transtentorial uncal herniation. Compression of the third cranial nerve is a more common and direct consequence.
* **Option B:** This option is not a typical consequence of transtentorial uncal herniation. Compression of the cerebral peduncle is more likely to cause ipsilateral hemiparesis rather than contralateral weakness.
* **Option D:** This option is not a typical consequence of transtentorial uncal herniation. Compression of the posterior cerebral artery can lead to ipsilateral occipital lobe ischemia, but this is not a direct consequence of the herniation.

**Clinical Pearl / High-Yield Fact**
In transtentorial uncal herniation, the "Cushing's triad" can be seen, which consists of increased blood pressure, irregular breathing, and bradycardia. This triad is a classic sign of increased intracranial pressure and brainstem compression.

**Correct Answer: A.**