What clinical sign is characteristic of Marek's disease in chickens?

• A. Enlarged nerves esp. sciatic
• B. Respiratory distress
• C. Diarrhea
• D. Feathering abnormalities

Answer: (A)

The main concept here is that Marek's disease causes a distinctive nervous-system manifestation. The virus induces lymphoid tumors and infiltration in nerve tissues, leading to enlargement of peripheral nerves. This neuropathy most commonly shows up as paralysis or weakness of the legs and wings, with the sciatic nerve often being noticeably enlarged. So, the characteristic clinical sign is enlarged nerves, especially the sciatic nerve, which points to Marek's disease.

Respiratory distress, diarrhea, and feathering abnormalities are not typical hallmarks of Marek's disease. They can occur with other poultry diseases or conditions, but they do not define the neuropathic presentation that Marek's disease classically causes.