**Core Concept**
Kaposi sarcoma is a type of cancer that primarily affects the skin and mucous membranes, characterized by the proliferation of spindle-shaped cells. It is a multifactorial disease, often linked to human herpesvirus 8 (HHV-8) infection and compromised immune systems.

**Why the Correct Answer is Right**
Kaposi sarcoma typically presents as painless, bluish-purple lesions, most commonly found on the lower limbs, particularly on the legs and feet. This predilection for the lower limbs is thought to be related to the high concentration of lymphatic vessels in this area, which provides a fertile ground for the tumor cells to proliferate. The lesions can also occur on the oral mucosa, genital mucosa, and other areas, but the lower limbs are the most frequently affected site.

**Why Each Wrong Option is Incorrect**
**Option A:** Upper limbs are less commonly affected by Kaposi sarcoma, although lesions can occasionally be found on the hands and arms.
**Option C:** Head and Neck involvement is rare in Kaposi sarcoma, with most cases presenting as oral or genital lesions.
**Option D:** Trunk involvement is possible, but the lesions are much less common on the trunk compared to the lower limbs.

**Clinical Pearl / High-Yield Fact**
Kaposi sarcoma is often associated with human immunodeficiency virus (HIV) infection, and its incidence has decreased significantly with the advent of antiretroviral therapy.

✓ Correct Answer: B. Lower limbs