A 7-year old castrated male Thoroughbred horse has a three-week history of intermittent cough associated with eating, unilateral (left-sided) Horner's syndrome, and intermittent bilateral, green-tinged nasal discharge. Two days ago, the horse bled profusely from the left nostril. Which of the following is the most likely source of the hemorrhage?
a. Ethmoid hematoma
b. Internal carotid artery
c. Maxillary sinus mucosa
d. Maxillary vein
e. Nasal turbinates