"皮肤的硬结组织" can be translated as "indurated tissue of the skin" or "skin induration" (where "induration" refers to the hardening of tissue).
"Indurated tissue of the skin" emphasizes the hardened tissue specifically in the skin.
"Skin induration" is a more concise term often used in medical contexts to describe this condition.
Both are acceptable, depending on the context and desired level of detail.