Carbon monoxide poisoning.

This is because the client reports having a headache and vertigo after turning on his furnace for the first time this season. These are common symptoms of carbon monoxide poisoning, which can occur when carbon monoxide, a colorless and odorless gas, builds up in the blood due to exposure to combustion fumes.

The furnace may have been a source of carbon monoxide if it was not well vented or maintained.

The other statements are wrong because:

Meniere’s disease is a disorder of the inner ear that causes vertigo, hearing loss, tinnitus and ear fullness. It is not related to carbon monoxide exposure or furnace use.

Migraine is a type of headache that causes throbbing pain, nausea, sensitivity to light and sound, and sometimes visual disturbances. It is not caused by carbon monoxide exposure or furnace use, but by genetic and environmental factors.

Benign paroxysmal positional vertigo (BPPV) is a condition that causes brief episodes of vertigo triggered by changes in head position. It is caused by tiny crystals in the inner ear that become dislodged and stimulate the balance sensors. It is not related to carbon monoxide exposure or furnace use.