## **Core Concept**
Sarcoidosis is a **multisystem granulomatous disease** characterized by the formation of non-caseating granulomas in various organs, most commonly the lungs and lymph nodes. The etiology of sarcoidosis remains unknown, but it is believed to result from a combination of genetic susceptibility and environmental triggers. The disease can affect any organ but most frequently involves the lungs, skin, and eyes.

## **Why the Correct Answer is Right**
To determine the correct answer, let's evaluate each statement:
1. **Most commonly involved organ is lung**: This statement is true. The lungs are the most frequently affected organ in sarcoidosis, with pulmonary involvement occurring in about 90% of patients.
2. **Lupus pernio is a type of sarcoid skin lesion**: This statement is true. Lupus pernio is a chronic form of sarcoidosis that presents with indurated, violaceous plaques or nodules on the face, particularly on the nose, cheeks, lips, forehead, and ears.
3. **90 % patients will have abnormal chest X-ray sometime during the course**: This statement is true. The majority of patients with sarcoidosis will have abnormal chest radiographs at some point during their disease course, reflecting pulmonary involvement.
4. **Blindness is not a complication**: This statement is false. Blindness can indeed be a complication of sarcoidosis, particularly if the disease affects the eyes (ocular sarcoidosis), leading to uveitis or other forms of ocular inflammation.
5. **Tissue biopsy is not mandatory to establish the diagnosis**: This statement is generally false. While not always necessary in every case (e.g., in Löfgren's syndrome, a specific form of acute sarcoidosis with characteristic symptoms and findings), tissue biopsy showing non-caseating granulomas is crucial for confirming the diagnosis in most cases.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because it does not accurately reflect the correct combination of true statements.
- **Option B:** This option is incorrect for similar reasons; it does not correctly identify the right combination of statements.
- **Option D:** This option suggests that all statements are correct, which is incorrect because statement 4 ("Blindness is not a complication") is false.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that **Löfgren's syndrome**, a subtype of sarcoidosis, presents acutely with **erythema nodosum, bilateral hilar lymphadenopathy on chest X-ray, and joint symptoms**. This syndrome often has a more favorable prognosis and may not require biopsy for diagnosis due to its characteristic presentation.

## **Correct Answer: C.**