**Core Concept**
Kaposi's Sarcoma (KS) is a type of cancer that affects the skin and mucous membranes, commonly in individuals with weakened immune systems, such as those with HIV/AIDS. The primary concern with KS is its potential to spread and cause life-threatening complications.

**Why the Correct Answer is Right**
The most common cause of death in Kaposi's Sarcoma is indeed massive pulmonary hemorrhage. This occurs when the cancerous cells infiltrate and damage the blood vessels in the lungs, leading to uncontrollable bleeding. The pathophysiology involves the proliferation of spindle cells, which are the characteristic cells of KS, and their ability to invade and disrupt the normal architecture of the lungs. The resulting hemorrhage can be catastrophic and often proves fatal.

**Why Each Wrong Option is Incorrect**
**Option A:** Dissemination, while a concern in KS, is not the most common cause of death. Dissemination refers to the spread of cancer cells to other parts of the body, which can lead to various complications, but it is not as immediately life-threatening as massive pulmonary hemorrhage.

**Option B:** AIDS is the underlying condition that predisposes individuals to KS, but it is not the cause of death in KS. AIDS is a systemic infection that compromises the immune system, making it more susceptible to KS and other opportunistic infections.

**Option D:** Diabetes mellitus is not directly related to KS or its complications. While individuals with diabetes may have compromised immune systems, diabetes itself is not a cause of death in KS.

**Clinical Pearl / High-Yield Fact**
KS is often associated with a 'bats wing' or 'panda eye' appearance due to the characteristic lesions on the skin. This distinctive appearance should raise suspicion for KS in individuals with a history of immunosuppression.

**Correct Answer Line**
✓ Correct Answer: C. Massive pulmonary haemorrhage