**Core Concept**
Kawasaki's disease is a medium vessel vasculitis characterized by inflammation of the blood vessels, particularly those in the coronary arteries, skin, and mucous membranes. It primarily affects children under the age of 5 and is a leading cause of acquired heart disease in this age group.

**Why the Correct Answer is Right**
Kawasaki's disease is characterized by a triad of clinical features: fever, rash, and lymphadenopathy. The rash is typically polymorphic, evolving through several stages, and is often accompanied by conjunctivitis and changes in the lips and oral mucosa. The disease is also associated with coronary artery aneurysms and cardiac complications.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as Kawasaki's disease is not typically associated with joint inflammation. While some patients may experience mild joint pain, significant joint involvement is not a hallmark of the disease.
**Option B:** This option is incorrect as Kawasaki's disease is not characterized by a high white blood cell count. In fact, patients often present with a normal or slightly elevated white blood cell count, but not the leukocytosis typically seen in bacterial infections.
**Option C:** This option is incorrect as Kawasaki's disease is not primarily an autoimmune disease. While the exact cause is unknown, it is believed to be triggered by an infectious agent, leading to an inflammatory response and vasculitis.

**Clinical Pearl / High-Yield Fact**
Kawasaki's disease is often referred to as "mimic" because of its nonspecific presentation and potential for misdiagnosis. A high index of suspicion and awareness of the classic clinical features are essential for early diagnosis and treatment.

**Correct Answer: D.**