A 3-month-old child has a moderate fever and non-productive cough and mild dyspnea. After a course of mild antibiotic, the condition of the child improved transiently but he again develops a high fever, productive cough and increased respiratory distress. Chest X-ray shows hyper lucency and PFT shows an obstructive pattern. Most probable diagnosis is –

**Core Concept**
The underlying principle being tested involves the diagnosis of respiratory conditions in infants, specifically those that present with initial improvement followed by worsening symptoms. This scenario suggests a condition that may have been partially responsive to antibiotics but then progressed, indicating a possible viral or secondary bacterial infection, or an obstructive airway disease.

**Why the Correct Answer is Right**
Given the clinical presentation of initial mild symptoms, transient response to antibiotics, and subsequent worsening with productive cough, high fever, and increased respiratory distress, along with radiographic and pulmonary function test (PFT) findings of hyperlucency and an obstructive pattern, the diagnosis is likely related to an airway condition. The hyperlucency on chest X-ray and obstructive pattern on PFT are key indicators of airway obstruction.

**Why Each Wrong Option is Incorrect**
**Option A:** This choice would not typically present with hyperlucency on a chest X-ray or an obstructive pattern on PFT.
**Option B:** Similar to Option A, it does not align with the specific radiographic and PFT findings described.
**Option C:** This option does not match the clinical progression or the specific diagnostic findings of hyperlucency and obstructive PFT pattern.

**Clinical Pearl / High-Yield Fact**
In infants presenting with respiratory symptoms that worsen after initial improvement, considering conditions that cause airway obstruction is crucial. The combination of clinical symptoms, transient response to antibiotics, and specific diagnostic findings like hyperlucency on chest X-ray and obstructive pattern on PFT can guide towards the correct diagnosis.

**Correct Answer:** D. Bronchiolitis.

✓ Correct Answer: D. Bronchiolitis obliterans