**Core Concept**
Mouth breathing in children is often associated with adenotonsillar hypertrophy, leading to nasal obstruction and subsequent changes in breathing patterns.

**Why the Correct Answer is Right**
The enlargement of adenoids and tonsils causes nasal obstruction, which forces the child to breathe through their mouth. This can lead to a range of issues, including sleep disturbances, speech difficulties, and growth problems. The nasal cycle, which normally alternates between the two nasal passages, is disrupted, and the nasal mucosa becomes dry and congested.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not a direct consequence of mouth breathing with enlarged adenoids and tonsils.
**Option B:** While mouth breathing can lead to changes in facial structure, it is not the most direct description of the condition.
**Option C:** This option is related to the condition, but it is not the most accurate description of mouth breathing with enlarged adenoids and tonsils.

**Clinical Pearl / High-Yield Fact**
Children with persistent mouth breathing and adenotonsillar hypertrophy are at risk of developing long-term changes in facial growth and development.

**Correct Answer: B. Nasal obstruction.**