Right side
真 supplies the sound: zhēn → shèn with regular zh-/sh- alternation.
真 also adds direct semantic flavour — being prudent is being TRUE to careful judgement, treating each step seriously. Same phonetic in
镇 (zhèn, town),
填 (tián, fill),
颠 (diān, top) — the
真 cluster spans zh-/sh-/t-/d- readings due to deep Old Chinese variation.