**Core Concept**
The "hummingbird sign" in brain MRI refers to the appearance of a hyperintense signal on T2-weighted images in the mesencephalic cisterns, which is indicative of increased CSF flow due to obstruction of CSF pathways. This phenomenon is often associated with Chiari malformation.

**Why the Correct Answer is Right**
The hummingbird sign is caused by the downward displacement of the cerebellar tonsils through the foramen magnum, leading to the obstruction of CSF flow through the fourth ventricle and the subsequent accumulation of CSF in the mesencephalic cisterns. This results in the characteristic "hummingbird" appearance on T2-weighted MRI images. The correct answer is associated with Chiari malformation, which is a congenital condition characterized by the herniation of the cerebellar tonsils through the foramen magnum.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the characteristic appearance of the hummingbird sign on T2-weighted MRI images. The sign is not typically associated with increased CSF flow in the cerebral ventricles.
**Option B:** This option is incorrect because it does not accurately describe the underlying cause of the hummingbird sign. The sign is not typically associated with hydrocephalus, but rather with Chiari malformation.
**Option C:** This option is incorrect because it does not accurately describe the characteristic appearance of the hummingbird sign on T2-weighted MRI images. The sign is not typically associated with a hyperintense signal in the cerebral cortex.

**Clinical Pearl / High-Yield Fact**
The hummingbird sign is a characteristic MRI finding that should raise suspicion for Chiari malformation, which can lead to a range of clinical symptoms including headaches, neck pain, and cerebellar dysfunction.

**Correct Answer: A. Chiari malformation.**