Which of the following parts of the ear does NOT contain endolymph?

Analyze the question: You are asked which part of the ear does not contain endolymph.

Explain the concept: The ear has three main regions:

• Outer ear: pinna and auditory canal
• Middle ear: tympanum (the middle ear cavity, bounded by the eardrum), ear ossicles, and Eustachian tube. It is filled with air.
• Inner ear: the membranous labyrinth and the cochlea.

The inner ear has two fluids:

• Endolymph is inside the membranous labyrinth. It fills the semi-circular canals (membranous part), the utriculus, the sacculus, and the cochlear duct.
• Perilymph surrounds the membranous labyrinth, between it and the bony labyrinth.

Step-by-step solution:

• A. Semi-circular canals: the membranous canals contain endolymph. ✔
• B. Utriculus: part of the membranous labyrinth, so it contains endolymph. ✔
• C. Tympanum: the middle ear cavity, which contains air and not endolymph. ✘
• D. Sacculus: part of the membranous labyrinth, so it contains endolymph. ✔

The tympanum is the only structure listed that is not part of the inner ear, so it is the one without endolymph.

Answer: C. Tympanum

Exam tip: Remember "Endolymph is Enclosed in the membranous labyrinth (inner ear)." The middle ear holds air, and the outer ear also holds air. If an option names a middle-ear or outer-ear structure, it will not contain endolymph.