Pertaining to within the muscles.

• A. Endocarditis
• B. Endomysium
• C. Endoderm
• D. Endometriosis

Answer: (B)

The concept being tested is the naming of tissue layers related to muscle anatomy. Endomysium is the tissue layer that lies within the muscle, wrapping around each individual muscle fiber. The prefix endo- means within, and mys (from myos) means muscle, with -ium denoting a connective tissue layer. So endomysium literally describes the layer inside the muscle around each fiber.

This is the best fit because the question asks for something pertaining to inside the muscles, and the endomysium is precisely the internal connective tissue sheath of a muscle fiber.

The other terms refer to different inside locations: endocarditis is infection of the inner lining of the heart, endoderm is the inner germ layer in the embryo, and endometriosis is growth of endometrial tissue outside the uterus.