**Core Concept**
Acute respiratory distress syndrome (ARDS) in children is a life-threatening condition characterized by rapid onset of hypoxemic respiratory failure. It is a clinical diagnosis that requires careful evaluation of the child's respiratory and cardiovascular status.

**Why the Correct Answer is Right**
The diagnosis of ARDS is based on the Berlin definition, which includes four criteria: (1) acute onset, (2) bilateral opacities on chest imaging, (3) respiratory failure not fully explained by cardiac failure or fluid overload, and (4) hypoxemia. In children, the PaO2/FiO2 ratio is often used to assess hypoxemia. A ratio of ≤ 300 mmHg is suggestive of ARDS.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the acute onset of respiratory distress, which is a key criterion for diagnosing ARDS.
**Option B:** This option is incorrect because it does not mention the bilateral opacities on chest imaging, which is a critical component of the Berlin definition.
**Option C:** This option is incorrect because it does not exclude cardiac failure or fluid overload as causes of respiratory failure, which is a requirement for diagnosing ARDS.

**Clinical Pearl / High-Yield Fact**
A PaO2/FiO2 ratio of ≤ 300 mmHg is a sensitive indicator of hypoxemia in children with ARDS. However, a ratio of > 300 mmHg does not exclude the diagnosis, as it may be seen in other conditions.

**Correct Answer: C. A PaO2/FiO2 ratio of ≤ 300 mmHg, bilateral opacities on chest imaging, acute onset, and respiratory failure not fully explained by cardiac failure or fluid overload.**