**Core Concept**
The clinical presentation of an ipsilateral third nerve lesion and contralateral ataxia is related to the **anatomy of the brainstem** and the **cerebellopeduncular pathways**. The third cranial nerve, or **oculomotor nerve**, controls most of the eye's movements, while ataxia is related to **cerebellar function**.

**Why the Correct Answer is Right**
Given the lack of specific options, the general approach to this question involves understanding that a lesion affecting the third nerve and causing contralateral ataxia would likely be located in the **brainstem**, specifically where the third nerve originates and where cerebellar peduncles connect to the brainstem.

**Why Each Wrong Option is Incorrect**
**Option A:** Without a specific description, we can't directly address why it's incorrect, but typically, options that don't align with brainstem pathology would be incorrect.
**Option B:** Similarly, without specifics, any option not implicating the brainstem or a structure that could cause both ipsilateral third nerve palsy and contralateral ataxia would be wrong.
**Option C:** And **Option D:** follow the same logic, being incorrect if they don't correctly identify a lesion that could cause these specific, crossed neurological signs.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that **crossed signs** (like an ipsilateral third nerve lesion with contralateral ataxia) often indicate a **brainstem lesion**, as the brainstem is where many cranial nerves originate and where cerebellar connections are made.

**Correct Answer:** D. Brainstem lesion