Final answer:
The most common symptom of feline viral rhinotracheitis is oculonasal discharge, reflecting the inflammatory response often seen in viral respiratory tract infections.
Step-by-step explanation:
The most common symptom of feline viral rhinotracheitis is oculonasal discharge. This illness, which mostly affects cats, is a type of upper respiratory infection. Similar to the human common cold, symptoms are often related to the body's immune response to the virus and can involve the lining of the nose, manifesting as nasal discharge, and the eyes, showing as ocular discharge. This is reflective of the inflammatory response in the mucosa that also occurs in human viral respiratory tract infections, leading to symptoms like runny nose and congestion. However, unlike humans, vomiting and diarrhea are less commonly associated with feline upper respiratory infections, whereas oculonasal discharge is a notable sign in cats afflicted with the disease.