혈전이 · 혈전을
blood clot, thrombus
Example
오래 앉아 있으면 다리에 혈전이 생길 수 있어요.
Sitting for a long time can cause blood clots in your legs.
Word parts
혈 blood+전 plug
Literally 'blood plug' — a clot that stoppers the vessel.
Good to know
Don't confuse it with 혈전 meaning 'bloody battle' (血戰) — same sound, different word.