結核
けっかく · kekkaku · JLPT N1 · noun
tuberculosis
核 — wood 木 + 亥 (phonetic)
A semantic-phonetic compound: 木 ('wood/tree') gives the meaning, while 亥 supplies the sound. It originally referred to a hard-shelled fruit pit or kernel inside wood/fruit, then extended figuratively to 'core,' 'nucleus,' and in modern usage 'nuclear' (as in atomic nucleus).