**Core Concept**
In the context of respiratory medicine, blood-stained sputum can be a symptom of various conditions. The key concept here is that some conditions can present with isolated hemoptysis (coughing up blood), which may be the only symptom.

**Why the Correct Answer is Right**
Hemoptysis can be a symptom of lung cancer, particularly non-small cell lung cancer (NSCLC). In NSCLC, tumor cells can invade the bronchial arteries or the pulmonary vessels, leading to bleeding. The cancer cells can also produce factors that stimulate angiogenesis (the formation of new blood vessels), further increasing the risk of bleeding. The presentation of isolated hemoptysis is often non-specific and may not be immediately associated with lung cancer.

**Why Each Wrong Option is Incorrect**
**Option A:** While tuberculosis (TB) can cause hemoptysis, it is usually associated with other symptoms such as cough, fever, and weight loss.
**Option B:** Pneumonia can cause hemoptysis, but it is typically associated with other symptoms such as fever, cough, and shortness of breath.
**Option C:** Asthma can cause hemoptysis, but it is usually associated with other symptoms such as wheezing, cough, and shortness of breath.

**Clinical Pearl / High-Yield Fact**
In patients presenting with isolated hemoptysis, a thorough investigation, including imaging and histopathological examination, is essential to rule out underlying malignancy, especially lung cancer.

**Correct Answer:** C. Lung cancer.