**Core Concept**
The underlying condition in this scenario is likely a pulmonary complication in an HIV-positive child, specifically Pneumocystis jirovecii pneumonia (PCP), which is characterized by bilateral interstitial infiltrates on chest X-ray and can present with respiratory distress.

**Why the Correct Answer is Right**
The child's presentation of sudden onset of breathlessness, hyperinflation on chest X-ray, and low oxygen saturation is consistent with PCP. The high O2 saturation in this case is not typical for PCP, which usually presents with significant hypoxemia and hyperinflation due to the accumulation of airspaces. However, some cases can present with relatively preserved oxygen saturation, especially if the disease is mild.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not provided. Please ensure that all options are given for accurate explanation.
**Option B:** This option is likely incorrect as it does not align with the typical treatment for PCP in HIV-positive children.
**Option C:** This option is incorrect as it does not address the specific treatment for PCP in HIV-positive children.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that PCP can present with a high O2 saturation in some cases, making it crucial to consider the diagnosis in HIV-positive children with respiratory symptoms, even if oxygen saturation is not severely low.

**Correct Answer: B. Trimethoprim-sulfamethoxazole (TMP-SMX)**