**Core Concept**
Myositis ossificans progressive (Fibrodysplasia ossificans progressiva, FOP) is a rare genetic disorder characterized by the gradual replacement of muscles, tendons, and other soft tissues with bone. This leads to progressive deformity and disability.

**Why the Correct Answer is Right**
The correct answer is lung disease because patients with FOP often develop restrictive lung disease due to chest wall deformities, which can lead to respiratory failure. The abnormal bone growth in the chest wall restricts lung expansion, making it difficult to breathe. This can lead to pneumonia, respiratory failure, and ultimately, death. The abnormal bone growth also increases the risk of fractures and subsequent respiratory complications.

**Why Each Wrong Option is Incorrect**
**Option A:** Nutritional deficiency is not a direct cause of death in FOP patients. While malnutrition may be a consequence of the disease due to difficulty eating and swallowing, it is not the primary cause of death.

**Option B:** Bed sore (pressure ulcer) is a potential complication in FOP patients, particularly those with significant mobility impairment. However, it is not the usual cause of death in these patients.

**Option D:** Septicemia is a potential complication in FOP patients, particularly those with respiratory infections. However, it is not the most common cause of death in these patients.

**Clinical Pearl / High-Yield Fact**
FOP patients often develop a characteristic "hanging shoulder" deformity due to the replacement of the deltoid muscle with bone. This deformity can lead to respiratory complications and is a hallmark of the disease.

**Correct Answer Line**
✓ Correct Answer: C. Lung disease