What is the rare cystic lesion typically located at the anterior roof of the third ventricle near the foramen of Monro, often causing obstructive hydrocephalus?

Colloid cyst. These lesions are usually hyperdense on CT and may appear hyperintense on T1-weighted MRI. They do not enhance with contrast. Clinically, they can cause headaches, memory disturbances, or sudden death due to acute hydrocephalus. A common pitfall is mistaking them for intraventricular tumors or cysts, such as ependymal cysts or neurocysticercosis.