What is the most appropriate pharmacotherapy for a dog with high fever, lethargy, and small operculated eggs in feces?

The presence of small operculated eggs in the feces suggests a parasitic infection, particularly one associated with flukes if we consider the eggs' morphology. In dogs, fluke infections can be caused by organisms like Fasciola hepatica or other trematodes, which could lead to systemic illness, including fever and lethargy due to the body's response to the parasitic burden.

Doxycycline is effective against a variety of infections, including those caused by certain protozoa and bacteria, and it can offer supportive treatment against secondary infections that might complicate a parasitic infection. Additionally, some parasitic infections that doxycycline can help treat also present with systemic signs, making it clinically relevant in this scenario.

In contrast, while other medication options like Amoxicillin, Praziquantel, and Fenbendazole have their own specific applications, they do not directly address the likely underlying cause indicated by the symptoms and fecal findings in this case. Amoxicillin is primarily used for bacterial infections and would not directly target parasites. Praziquantel is specifically effective against cestodes (tapeworms) rather than trematodes, which would not be suitable given the egg morphology. Fenbendazole can be useful