Keratin is NOT found in which of the following locations?

Keratin is a fibrous structural protein that plays a crucial role in the composition of various parts of the integumentary system, primarily providing strength and resilience. It is a major component of hair, nails, and the outer layer of skin. Each of these structures relies on keratin for their protective properties and structural integrity.

The head, while it may contain hair and skin, is not a specific location where keratin itself resides as a distinct entity. Instead, it's a general area that encompasses multiple structures that do contain keratin but does not refer to a specific tissue or type of keratinized structure. In the context of this question, recognizing that hair, nails, and skin are all specific tissues with keratin as a primary component clarifies why "head" is not a suitable answer, as it does not represent a specific site of keratin presence.