Swimmer's ear is an infection primarily of which parts related to middle ear problems?

Swimmer's ear, also known as otitis externa, is an infection that affects the external ear canal. This condition is primarily caused by water remaining in the ear after swimming or bathing, which creates a moist environment that fosters bacterial growth. The key components involved are the external ear canal and the mechanisms that can contribute to the infection.

The Eustachian tube, while crucial for equalizing pressure in the middle ear and draining fluid, is not primarily involved in swimmer's ear. Instead, this condition stems from issues related specifically to the external ear canal where pathogens can enter and proliferate.

In this context, recognizing that the Eustachian tube can sometimes contribute to ear problems, it's the pairing with the external ear canal that reinforces the nature of swimmer's ear, distinguishing it from infections that affect the middle ear, which are more commonly associated with the Eustachian tube. Hence, option D appropriately identifies the relevant areas affected in swimmer's ear, aligning with the characteristics of the infection.