痧

Outer sickness radical, a stylized bed-on-its-side with a person leaning against it, picturing illness. The indexing radical, files 痧 in the medical family: 病 illness, 痛 pain, 疹 rash, 疾 disease, 疫 epidemic. Marks 痧 as a TCM disease category, the cluster of acute summer illnesses (heatstroke, cholera-like spasms) traditionally treated by scraping 刮痧.

Inner 沙 supplies the sound, shā exactly, no shift. Pure phonetic role; the 'sand' meaning contributes nothing semantically. Same phonetic family: 砂 sand-stone, 莎 sedge, 鲨 shark. The medical name 痧 likely picks up the phonetic via shared metaphor with the gritty, fine red spots (the rash sense in 痧子) but that semantic link is loose at best.