A 7-year-old castrated male thoroughbred horse has signs indicating a serious condition. What is the most likely source of the hemorrhage?

The most likely source of hemorrhage in this scenario is the internal carotid artery. This artery is a critical blood vessel that supplies blood to the brain and other important structures in the head. In horses, particularly if there are indications of serious conditions such as severe sinus infections, neoplasia, or trauma, the internal carotid artery can become compromised or erode due to the underlying pathology, leading to significant hemorrhage.

Looking at the other potential sources, an ethmoid hematoma usually causes localized bleeding but is generally less severe and more chronic. The maxillary sinus mucosa can also be involved in cases of sinusitis or neoplasms, but bleeding from here may not be as immediately life-threatening or extensive as that from the internal carotid artery. Nasal turbinates could be sources of nasal bleeding, often associated with mild to moderate conditions, but again, they are less likely to produce the severe hemorrhage seen in this case.

Given the presentation of signs indicating a serious condition, the internal carotid artery represents the most critical and likely source for hemorrhage, emphasizing the need for urgent investigation and management.