Meniere's disease primarily affects which structure in the ear?

Meniere's disease is primarily characterized by its impact on the inner ear, specifically affecting the cochlea and the vestibular system. The cochlea plays a crucial role in hearing, as it contains the hair cells that convert sound vibrations into neural signals that are sent to the brain. In patients with Meniere's disease, an accumulation of fluid in the inner ear disrupts the normal balance of pressure in the cochlea, leading to symptoms such as hearing loss, tinnitus (ringing in the ear), and a sensation of fullness.

While the semicircular canals, which are involved in balance, can also be affected due to the fluid buildup, the cochlea is directly responsible for the auditory dysfunction that defines the disease. Therefore, the correct answer points to the cochlea as the primary structure impacted in the context of Meniere's disease.