**Core Concept**
Sarcoidosis is a multisystem, granulomatous disorder characterized by the formation of non-caseating granulomas in various organs, including the lungs, lymph nodes, and skin. The clinical presentation of this patient, with symmetrical, red, tender swellings in the shin and hilar and paratracheal lymphadenopathy on X-ray, is suggestive of cutaneous and pulmonary involvement in sarcoidosis.

**Why the Correct Answer is Right**
The next step in the diagnosis of sarcoidosis is to confirm the presence of non-caseating granulomas, which are a hallmark of the disease. This can be achieved through a biopsy of the affected tissue, such as the skin or lymph node. The biopsy would be stained with histological dyes, such as hematoxylin and eosin, to visualize the granulomas. The presence of non-caseating granulomas would confirm the diagnosis of sarcoidosis.

**Why Each Wrong Option is Incorrect**
**Option A:** Blood tests, such as erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP), may be elevated in sarcoidosis, but they are not specific for the diagnosis and would not confirm the presence of non-caseating granulomas.

**Option B:** Chest X-ray and computed tomography (CT) scans may show hilar lymphadenopathy and pulmonary infiltrates in sarcoidosis, but these imaging studies would not provide a definitive diagnosis, as they may also be seen in other conditions, such as tuberculosis or lung cancer.

**Option C:** Skin punch biopsy would be a more specific approach to confirm the diagnosis of sarcoidosis, as it would allow for the examination of the affected skin tissue.

**Option D:** Serological tests, such as the angiotensin-converting enzyme (ACE) test, may be positive in sarcoidosis, but they are not specific and would not confirm the presence of non-caseating granulomas.

**Clinical Pearl / High-Yield Fact**
In sarcoidosis, the presence of non-caseating granulomas is diagnostic, and skin or lymph node biopsy is the most specific approach to confirm the diagnosis.

**Correct Answer:** C. Skin punch biopsy would be a more specific approach to confirm the diagnosis of sarcoidosis.