Which of the following is a clinical sign of Dermatophilus congolensis infection?

The clinical sign associated with a Dermatophilus congolensis infection is characterized by paintbrush lesions and crust scab formation. This bacterium causes a condition known as dermatophilosis, which typically presents with distinctive skin lesions. The "paintbrush" appearance refers to the way in which the scabs form and the hair tufts stick together, giving a unique visual resemblance to a paintbrush. Infected animals may exhibit these scabs along with other skin-related issues, making this option the most relevant to the infection caused by Dermatophilus congolensis.

The other choices reflect clinical signs associated with different medical conditions. For instance, fever and lethargy can be indicative of a variety of systemic infections, gastrointestinal distress is often linked to digestive disorders, and respiratory difficulty can arise from respiratory illnesses. None of these are specific to dermatophilosis, which is why they do not represent the primary clinical signs of an infection with Dermatophilus congolensis.