Complex sentence: It affects many organ systems, mainly those including the blood vessels, skin, mucous membranes, and lymph nodes; however, its rarest but most serious effect is on the heart, where it can cause fatal coronary artery aneurysms in untreated children.
Split into simpler sentences: It affects many organ systems, mainly those including the blood vessels, skin, mucous membranes, and lymph nodes. Its rarest but most serious effect is on the heart, where it can cause fatal coronary artery aneurysms in untreated children.