**Core Concept**
Transtentorial uncal herniation is a life-threatening condition where the uncus of the temporal lobe is displaced across the tentorial edge, compressing the third cranial nerve and leading to ipsilateral pupillary dilation. This occurs due to increased intracranial pressure, typically from a mass lesion or hemorrhage.

**Why the Correct Answer is Right**
In transtentorial uncal herniation, the uncus compresses the third cranial nerve, leading to ipsilateral pupillary dilation (Marcus Gunn pupil) and potentially causing contralateral hemiparesis due to compression of the cerebral peduncle. The compression of the midbrain also leads to the Kemic's notch sign on imaging, which is an indentation of the midbrain on the contralateral side.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is not a consequence of transtentorial uncal herniation. Cushing's triad (increased blood pressure, irregular breathing, and a reduction in heart rate) is typically seen in raised intracranial pressure but is not specific to transtentorial herniation.
**Option B:** This option is incorrect because it is a possible consequence of transtentorial uncal herniation. Coma and decreased consciousness can occur due to compression of the brainstem and increased intracranial pressure.
**Option C:** This option is incorrect because it is a possible consequence of transtentorial uncal herniation. Ipsilateral pupillary dilation (Marcus Gunn pupil) occurs due to compression of the third cranial nerve.

**Clinical Pearl / High-Yield Fact**
An important clinical correlation is that transtentorial uncal herniation typically occurs in the setting of a space-occupying lesion, such as a tumor or hemorrhage. Recognizing the early signs of transtentorial herniation, such as a dilated pupil on the same side as the lesion, is crucial for prompt intervention.

**Correct Answer:** D