Pacheco's disease in birds primarily causes which of the following clinical signs?

Pacheco's disease, an infectious viral disease caused by a herpesvirus, is known for causing acute mortality in infected birds, especially in species like psittacines (parrots). The clinical sign highlighted in the correct response involves the death of well-fleshed animals, which often presents with bright yellow urates. This particular symptom is significant as it indicates liver failure, a common complication in cases of Pacheco's disease. The yellow color is typically associated with excessive bilirubin in the urine due to liver dysfunction, which leads to a rapid decline in health and subsequent death in affected birds.

This manifestation supports the idea that Pacheco's disease has a high mortality rate and can lead to sudden death in otherwise healthy-looking birds. The other signs mentioned in the other options may occur in various avian diseases but are not characteristic indicators of Pacheco's disease specifically. Weight loss and lethargy, seizures and ataxia, as well as respiratory distress and nasal discharge, are more commonly associated with other pathological conditions in birds, rather than being hallmark signs of this particular viral infection.