**Core Concept**
Sarcoidosis is a systemic granulomatous disease characterized by the formation of non-caseating granulomas in various organs, including the lungs. Radiological findings in sarcoidosis are often subtle and non-specific, but can provide important clues for diagnosis.

**Why the Correct Answer is Right**
Bilateral hilar lymphadenopathy (BHL) is the most common and characteristic radiological finding in sarcoidosis. This is due to the infiltration of granulomatous tissue into the hilar lymph nodes, which can cause them to enlarge and become visible on chest X-rays or CT scans. BHL is often seen in the early stages of the disease and can be a key indicator of sarcoidosis, especially in patients with no symptoms or minimal symptoms.

**Why Each Wrong Option is Incorrect**
**Option A:** Parenchymal disease, such as fibrosis or honeycombing, can be seen in sarcoidosis, but it is not the most characteristic or early radiological finding. Parenchymal changes are often seen in more advanced or chronic cases.
**Option B:** Unilateral hilar lymphadenopathy can be seen in sarcoidosis, but it is less common than bilateral hilar lymphadenopathy. Unilateral BHL can also be seen in other conditions, such as tuberculosis or malignancy.
**Option D:** Miliary shadow is a radiological finding seen in conditions such as tuberculosis, silicosis, or military sarcoidosis, but it is not the characteristic finding in sarcoidosis. Miliary shadow is often seen in more advanced or disseminated cases.

**Clinical Pearl / High-Yield Fact**
The Heerfordt's sign, which is the presence of bilateral hilar lymphadenopathy with or without symptoms such as fever, weight loss, or fatigue, is a classic radiological finding in sarcoidosis. This sign is often seen in the early stages of the disease and can be a key indicator of sarcoidosis.

**✓ Correct Answer: C. Bilateral hilar lymphadenopathy**