**Core Concept:** Infections of the mastoid air cells are typically caused by bacteria entering the mastoid space, which is a potential space located posterior to the pinna of the ear and inferior to the parotid gland. Infections in this space can spread to the mastoid air cells, leading to mastoiditis.

**Why the Correct Answer is Right:** The correct answer, C (Space of Retzius), is a potential space located between the mastoid process and the parotid gland. Bacteria entering this space can spread to the mastoid air cells, leading to mastoiditis. Space of Retzius is an important space to understand as it is a common pathway of infection for mastoiditis.

**Why Each Wrong Option is Incorrect:**

A. Space of Eustachius (between the tympanic membrane and the Eustachian tube): This space is not related to the mastoid air cells and mastoiditis. The infection would typically enter the middle ear first before ascending to the mastoid air cells, making this option incorrect.

B. Space of Fallopius (between the tympanic membrane and the Eustachian tube): Similar to option A, this space is not related to mastoiditis. Infection would enter the middle ear before ascending to the mastoid air cells.

D. Space of Wrisberg (between the tympanic membrane and the Eustachian tube): This space is also not related to mastoiditis. The infection would typically enter the middle ear before ascending to the mastoid air cells.

**Clinical Pearl:** Mastoiditis is a clinical condition that can lead to complications if not treated promptly. It is essential to understand the anatomy of potential spaces such as Space of Retzius to identify and treat this infection appropriately.