**Core Concept**
Middle ear infections, also known as otitis media, can spread to the brain through various routes, leading to serious complications such as meningitis, brain abscess, or cerebellar abscess. The spread of infection is facilitated by the close proximity of the middle ear to the cranial cavity and the presence of emissary veins.

**Why the Correct Answer is Right**
The spread of infection from the middle ear to the brain primarily occurs through the following routes:

* **Option A:** Through the middle ear's emissary veins, which connect the middle ear to the sigmoid sinus and the cavernous sinus, allowing bacteria to travel to the brain. This route is facilitated by the presence of mastoid air cells and the petrous apex.
* **Option B:** Through the petrosal sinus, a venous structure that connects the middle ear to the cavernous sinus. Infection can spread through this route, particularly if the petrous apex is involved.
* **Option C:** Through the Eustachian tube, which connects the middle ear to the nasopharynx. Although this route is more commonly associated with the spread of infection from the nasopharynx to the middle ear, it can also facilitate the spread of infection from the middle ear to the cranial cavity.

**Why Each Wrong Option is Incorrect**
* **Option D:** Through the external auditory canal. Although external ear infections can spread to the middle ear, this route is not a common pathway for the spread of infection from the middle ear to the brain.

**Clinical Pearl / High-Yield Fact**
It is essential for clinicians to be aware of the potential for middle ear infections to spread to the brain, particularly in cases of mastoiditis or petrositis. Prompt recognition and treatment of these complications can help prevent serious outcomes.

**Correct Answer: D. Through the external auditory canal.**