inflammation, flame
炎（ほのお）、炎症（えんしょう）、火炎（かえん）
flame, inflammation, blaze
On-reading (エン) is used in compounds; kun-reading (ほのお) is used standalone or with okurigana. Appears in many compounds — memorize the key ones rather than the kanji in isolation.
From the JLP Quiz question bank — the word in the form it is actually tested in.
医師は検査の結果をもとに、患者を肺炎と（ ）した。
Answer: 診断
