**Core Concept**
Transtentorial herniation is a life-threatening condition where brain tissue is forcibly pushed across structures within the skull, resulting in **brainstem compression** and **cerebral ischemia**. This occurs due to increased **intracranial pressure**. The **uncal portion of the temporal lobe** is often implicated in this process.

**Why the Correct Answer is Right**
Transtentorial herniation can cause **ipsilateral hemiparesis** due to compression of the **cerebral peduncle** against the tentorium, **third nerve palsy** with **ipsilateral pupillary dilatation** due to compression of the **oculomotor nerve**, and **contralateral hemiparesis** is not typically a direct result of the herniation itself but can occur due to **posterior cerebral artery infarction**. Neck stiffness is more commonly associated with **meningitis**.

**Why Each Wrong Option is Incorrect**
**Option A:** Neck stiffness is not a direct result of transtentorial herniation.
**Option B:** Post cerebral artery infarct can occur due to transtentorial herniation.
**Option D:** 3rd Nerve palsy with ipsilateral pupillary dilatation is a known consequence.

**Clinical Pearl / High-Yield Fact**
Transtentorial herniation is a medical emergency requiring prompt intervention to reduce **intracranial pressure** and prevent further **brain injury**. Recognizing the signs of herniation, such as **third nerve palsy** and **ipsilateral pupillary dilatation**, is crucial for timely management.

**Correct Answer:** C. Contralateral hemiparesis