**Core Concept**
A foreign body in the nasal cavity is a common cause of unilateral (U/L) mucopurulent rhinorrhea in children due to the obstruction of the nasal passage and subsequent accumulation of mucus. This condition can lead to increased production of mucus, which becomes purulent due to secondary infection.

**Why the Correct Answer is Right**
The presence of a foreign body in the nasal cavity can cause mucopurulent rhinorrhea by obstructing the nasal passage and leading to a buildup of mucus. The foreign body can irritate the nasal mucosa, causing inflammation and increased mucus production. This, combined with secondary infection, results in the characteristic mucopurulent discharge. The nasal cavity's natural defense mechanisms, including the mucociliary escalator, are impaired by the foreign body, leading to the accumulation of mucus and subsequent infection.

**Why Each Wrong Option is Incorrect**

**Option B:** Adenoids blocking the airways is more commonly associated with obstructive sleep apnea, recurrent otitis media, or nasal congestion, but is not typically the cause of unilateral mucopurulent rhinorrhea.

**Option C:** A deviated nasal septum can cause nasal obstruction and congestion, but it is not typically associated with mucopurulent rhinorrhea.

**Option D:** Inadequately treated acute frontal sinusitis can cause purulent rhinorrhea, but it is usually bilateral and not unilateral.

**Clinical Pearl / High-Yield Fact**
In children, a unilateral mucopurulent rhinorrhea is often a red flag symptom for a foreign body in the nasal cavity, and prompt removal of the foreign body is essential to prevent complications such as nasal septal perforation or chronic sinusitis.

**✓ Correct Answer: A. Foreign body. A foreign body in the nasal cavity is a common cause of unilateral mucopurulent rhinorrhea in children.**