scrofulous swellings and sores
痄 refers to a specific medical condition: scrofulous swellings and sores, a type of tuberculous infection of the lymph nodes. Its meaning is narrow and clinical.
痄 combines the sickness radical 疒 with the phonetic component 乍. The exact historical development is uncertain, but it likely follows the common phono-semantic pattern of Chinese medical characters.
The sickness enclosure 疒 surrounds 乍, like a swelling or sore trapped under the skin. Picture a painful lump that won't go away.
For サ, imagine a sore that is so sudden it makes you say 'Sa!' in surprise. The sharp sound 'Sa' cues サ while the sore appears.
—