1

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.

3

Here it means:

"The flat working surface on top of waist-level kitchen cabinets."

See countertop

Here it is used as an item in a list of things that can easily be stained.

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