Anthralin is a topical medication used in treatment of psoriasis. I took the following text from online article.
Anthralin. This medication is believed to normalize DNA activity in skin cells. Anthralin (Dritho-Scalp) can also remove scale, making the skin smoother. However, anthralin can irritate skin, and it stains virtually anything it touches, including skin, clothing, countertops and bedding. For that reason doctors often recommend short-contact treatment — allowing the cream to stay on your skin for a brief time before washing it off. Anthralin is sometimes used in combination with ultraviolet light.
countertop means top surface of the basin, but it has no connection to the above context.