You will be given a trivia clue, and the category it belongs to. You should answer with the best answer that belongs in the category and is described by the clue. For simplicity, answers should be in all lower cased letters.

Category: ART & ARTISTS 
Clue: An X-ray of his "Blue Boy" has revealed the figure of a small dog that was painted out
thomas gainsborough

Category: GLAND FINALE 
Clue: This human gland important in the immune system takes its name in part from its resemblance to an herb
the thymus gland

Category: SIGNS & SYMBOLS 
Clue: The 2 symbols seen on pirate flags & bottles of poison

skull & crossbones