A 50-year-old male patient presented with weakness of B/L lower limbs which was symmetrical (distal weakness>> proximal muscle weakness) for 1 month. On examination, ceain skin and nail changes were noted. Patient also complained of myalgia and ahralgia. Lab findings, Raised creatinine kinase levels Raised ANA antibody titre Muscle biopsy was done from the quadriceps femoris muscle. CD4+ cells were found in the cellular infiltrate. IHC study of which protein is a very sensitive diagnostic test for the above condition: –

## **Core Concept**
The patient's presentation suggests an inflammatory muscle disease, specifically **polymyositis**, which is characterized by symmetrical distal muscle weakness, skin and nail changes, myalgia, arthralgia, and elevated creatine kinase (CK) levels. The presence of CD4+ cells in the muscle biopsy further supports this diagnosis.

## **Why the Correct Answer is Right**
The condition described is polymyositis, an **inflammatory myopathy** primarily affecting skeletal muscles. The diagnosis often involves a combination of clinical findings, laboratory tests (like elevated CK levels and positive ANA), and histopathological examination of muscle biopsy. The presence of CD4+ T cells in the muscle biopsy is indicative of an immune-mediated process. **Immunohistochemical (IHC) staining for membrane attack complex (MAC)**, also known as complement component 5b-9 (C5b-9), is highly sensitive for diagnosing polymyositis and dermatomyositis. MAC deposition on capillaries is an early and sensitive indicator of these conditions.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While certain proteins might be involved in muscle pathology, **troponin** is more commonly associated with cardiac muscle damage and not specifically used for diagnosing inflammatory myopathies.
- **Option B:** **Desmin** is an intermediate filament protein important in muscle structure, and its abnormalities can lead to myofibrillar myopathies, not typically used for diagnosing polymyositis.
- **Option D:** **Cytokeratin** is related to epithelial cell structure and is not directly relevant to the diagnosis of inflammatory muscle diseases like polymyositis.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that **polymyositis and dermatomyositis** can be distinguished from other myopathies by the presence of **characteristic skin manifestations** (like Gottron's papules in dermatomyositis) and **specific muscle biopsy findings**, including perifascicular atrophy and capillary deposition of MAC. Early diagnosis and treatment can significantly improve outcomes.

## **Correct Answer:** . **C5b-9 (MAC)**

✓ Correct Answer: A. Myxovirus resistance protein