40 years old male patient came with complaint of dry cough and dyspnoea. On lung function tests, restrictive pattern of decreases lung function is seen. Chest x-ray showed few reticular opacities in bilateral lung. HRCT of lung was performed which shows GGO in bilateral lung fields sparing sub-pleural region. Most probable diagnosis is

**Core Concept**
Non-specific interstitial pneumonia (NSIP) is a type of idiopathic interstitial pneumonia characterized by a uniform distribution of inflammation and fibrosis in the lung parenchyma, often presenting with a restrictive pattern on pulmonary function tests. The disease typically spares the sub-pleural regions, which is a key radiological feature used for diagnosis.

**Why the Correct Answer is Right**
The high-resolution computed tomography (HRCT) findings in this patient, including ground-glass opacities (GGO) in bilateral lung fields sparing the sub-pleural regions, are classic for NSIP. The uniform distribution of inflammation and fibrosis in NSIP is distinct from the heterogeneous pattern seen in usual interstitial pneumonia (UIP). The presence of GGO and the absence of honeycombing or traction bronchiectasis further support the diagnosis of NSIP. The clinical presentation of dry cough and dyspnea, combined with the radiological findings, makes NSIP the most probable diagnosis.

**Why Each Wrong Option is Incorrect**

**Option A:** UIP is characterized by a heterogeneous pattern of inflammation and fibrosis, often with honeycombing and traction bronchiectasis, which is not seen in this patient's HRCT.
**Option C:** Hypersensitivity pneumonitis typically presents with a more heterogeneous pattern on HRCT, often with upper lobe predominance and mosaic attenuation, which is not consistent with the findings in this patient.
**Option D:** Respiratory bronchiolitis is a type of idiopathic interstitial pneumonia that typically presents with a peribronchiolar pattern of inflammation and fibrosis, which is not seen in this patient's HRCT.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the sub-pleural regions are often spared in NSIP, which can help differentiate it from UIP. This radiological feature, combined with the uniform distribution of inflammation and fibrosis, is a key diagnostic clue for NSIP.

**Correct Answer:**
✓ Correct Answer: B. NSIP

✓ Correct Answer: B. NSIP