scab, dry up, slough
痂 centers on the hardened crust that forms over a healing wound: a scab. The related senses of drying up and sloughing off describe the natural progression of a scab as it dries and eventually falls away.
痂 combines 疒 (sickness) with 加, which likely contributes the sound and may suggest the idea of something added or attached to the skin, like a scab forming over a wound.
The sickness enclosure 疒 surrounds 加 (add). Picture a wound adding a protective crust: a scab that dries up and eventually sloughs off.
For カ, imagine a car with a scab-like patch of rust on its door: car -> カ, and the rust patch dries up and flakes away like a scab.