**Question:** Eustachian tube opens -

A. During swallowing
B. When the eardrum is stretched
C. Upon inspiration
D. When the Valsalva maneuver is performed

**Core Concept:**
The Eustachian tube is a one-way respiratory passage that connects the middle ear to the nasopharynx. Its primary function is to equalize the pressure between the middle ear and the atmosphere, and to drain secretions from the middle ear.

**Why the Correct Answer is Right:**
The Eustachian tube opens during the Valsalva maneuver (Option D) due to increased intraocular pressure, which forces the eardrum to bulge and open the tube. This allows air to enter the middle ear and equalize the pressure.

**Why Each Wrong Option is Incorrect:**

A. During swallowing (Option A) is incorrect because swallowing primarily involves the pharynx and esophagus, not the Eustachian tube.
B. When the eardrum is stretched (Option B) is not the primary mechanism for Eustachian tube opening, as the tube opens due to increased pressure, not eardrum stretch.
C. Upon inspiration (Option C) is not the primary mechanism as the Eustachian tube primarily opens in response to increased pressure, not respiratory changes.

**Clinical Pearl / High-Yield Fact:**
The Eustachian tube plays a crucial role in maintaining middle ear pressure balance and preventing middle ear infections (otitis media). A dysfunction of the Eustachian tube can lead to recurrent ear infections and tympanic membrane perforations.

**Correct Answer:** D. When the Valsalva maneuver is performed.