**Question:** A 30-year-old woman has had gradually increased muscle weakness with myalgia for the past year. She now has difficulty getting up from a chair and climbing stairs. She does not have weakness in her hand muscles. Physical examination reveals a fine violaceous rash on her face, predominantly palpebral. Dusky, flat, red patches are present on her elbows, knees, and knuckles. Laboratory studies show serum creatine kinase of 620 U/L. A deltoid biopsy specimen is obtained, and on microscopic examination shows a mononuclear inflammatory cell infiltrate around small blood vessels and groups of atrophic myofibers at the periphery of fascicles. What mechanism is most likely responsible for her disease?

A. Autoimmune myositis
B. Drug-induced myositis
C. Neoplastic myositis
D. Infectious myositis

**Correct Answer:** C. Neoplastic myositis

**Core Concept:** Neoplastic myositis is a muscle inflammation caused by the presence of a neoplastic disease, typically lymphoma or leukemia. The high serum creatine kinase levels (620 U/L) and the characteristic biopsy findings of mononuclear inflammatory cell infiltrate around small blood vessels and groups of atrophic myofibers at the periphery of fascicles, suggestive of tissue damage, are consistent with this diagnosis.

**Why the Correct Answer is Right:**

1. **Neoplastic myositis**: Myositis (muscle inflammation) occurs as a result of an immune response against a neoplastic disease, usually lymphoma or leukemia. This is distinct from the other options:

- **Autoimmune myositis** (option A) involves an autoimmune response against muscle proteins, leading to muscle inflammation. This is typically seen in conditions like dermatomyositis or polymyositis.
- **Drug-induced myositis** (option B) is caused by the administration of certain medications, such as statins or NSAIDs, leading to muscle inflammation. This is not consistent with the given case.
- **Infectious myositis** (option D) is caused by a direct muscle infection, which is not present in this case.

2. **Why the Correct Answer is Not Right**:

- **Neoplastic myositis** (option C) is a specific type of myositis caused by the immune response against a neoplastic disease, such as lymphoma or leukemia. In this case, the patient has a history of gradually increasing muscle weakness and myalgia, with no involvement of hand muscles. Additionally, the biopsy findings highlight the presence of atrophic myofibers at the periphery of fascicles, which is characteristic of neoplastic myositis.

3. **Why the Wrong Answers are Right:**

- **Autoimmune myositis (option A)** typically involves hand muscles and can be associated with cutaneous manifestations like Gottron's papules.
- **Drug-induced myositis (option B)** is typically caused by medications like statins or nonsteroidal anti-inflammatory drugs (NSAIDs). The given