Given a category and a set of five words, find the word from the set that does not belong (i.e. is the least relevant) with the other words in the category. Words are separated by commas.

Q: Category: physical property 
Words: brittleness, protein, bulky, skinny, fat

A: protein
****
Q: Category: aesthetic characteristic 
Words: loyal, medium, ghostly, tone, fair

A: loyal
****
Q: Category: body part 
Words: arm, kilometer, elbow, heel, foot

A:
kilometer
****