In a dog suspected of having a drug reaction, which skin lesion type should be biopsied?

In cases of drug reactions, pustules are the recommended type of skin lesion to biopsy. Pustules are small, raised bumps filled with pus that can indicate an inflammatory or infectious process. These lesions are often associated with various dermatological conditions, including those caused by adverse drug reactions. Biopsying pustules can provide valuable histological information, as they can reveal the presence of neutrophils and provide insight into the underlying immunological processes involved in the drug reaction.

Crusts, while they may indicate secondary infection or inflammation, generally do not yield as much diagnostic information as pustules concerning the acute inflammatory response. Erosions are generally superficial losses of the epidermis, which may not provide a clear picture of the underlying pathology. Similarly, ulcers involve deeper tissue damage and may not reflect the initial inflammatory response typical of a drug reaction. Therefore, although all these lesions can be relevant in the context of a drug reaction, pustules provide the best opportunity for diagnosis and understanding the underlying mechanism at play.