## **Core Concept**
The question tests the knowledge of common causes of unilateral nasal obstruction and foul-smelling nasal discharge in children. This presentation is suggestive of a condition that causes a significant difference in nasal airflow and possibly an infection or foreign body. The key concept here involves understanding the differential diagnoses for unilateral nasal symptoms.

## **Why the Correct Answer is Right**
The correct answer, **D. Nasal foreign body**, is the most likely diagnosis given the symptoms of unilateral nasal obstruction with foul-smelling nasal discharge in a child. Children often insert small objects into their nostrils, which can cause unilateral obstruction. The presence of a foreign body can lead to infection, resulting in foul-smelling discharge. This condition is a common pediatric emergency and requires prompt diagnosis and treatment.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is blank and cannot be evaluated.
- **Option B:** This option is blank and cannot be evaluated.
- **Option C:** This option is blank and cannot be evaluated.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that in children presenting with unilateral nasal obstruction, a thorough examination of the nasal passages is crucial, and a nasal foreign body should always be considered and ruled out. The classic presentation can be remembered by the "unilateral" nature of symptoms and the possibility of a "foreign" object being the cause.

## **Correct Answer: D. Nasal foreign body**