anal fistula, tumor, sore
瘺 centers on abnormal growths or lesions on the body: an anal fistula, a tumor, or a sore. The common thread is a localized, often painful, pathological swelling or opening.
瘺 combines the sickness radical 疒 with the phonetic component 屚, which likely contributes the sound and may relate to leaking or oozing, fitting the fistula sense. The exact historical development is uncertain.
Under the sickness roof 疒, the inner shape 屚 looks like a body with a leaking sore. Picture a painful, swollen spot that won't heal.
For ル, imagine a rude, leaking sore that makes you say 'Eww, rude!' — 'rude' cues ル, and the sore is the fistula.