**Core Concept**
The endolymphatic duct is a key structure in the inner ear that drains endolymph from the cochlea and vestibular system into the vestibular aqueduct, ultimately draining into the subarachnoid space via the endolymphatic sac. This pathway is critical for maintaining ionic balance and fluid homeostasis in the inner ear.

**Why the Correct Answer is Right**
The endolymphatic duct drains endolymph from the inner ear into the endolymphatic sac, which then connects to the **subarachnoid space** through the foramen of the internal auditory canal. The subarachnoid space is a potential space between the arachnoid and pia mater, and it receives cerebrospinal fluid (CSF) and, indirectly, drainage from the inner ear via the endolymphatic system. The duct does not drain into the subdural space (which lies beneath the dura mater) or external spaces. The term "succulus" refers to a thin membrane in the inner ear, not a drainage destination.

**Why Each Wrong Option is Incorrect**
Option B: The external space (e.g., external auditory canal) is outside the bony labyrinth and not a drainage pathway for endolymph.
Option C: The subarachnoid space is correct in function but the **endolymphatic duct does not directly drain into it**—it drains via the endolymphatic sac into the subarachnoid space through the internal auditory canal, making this choice misleading.
Option D: "Succulus" refers to the thin membrane of the inner ear (e.g., succus of the cochlea), not a drainage pathway.

**Clinical Pearl / High-Yield Fact**
The endolymphatic duct and sac are vital in regulating endolymph volume; obstruction or dysfunction can lead to Ménière’s disease, characterized by vertigo, hearing loss, and tinnitus.

✓ Correct Answer: A. Subdural space