Correct spelling for the English word "pexin" is [pˈɛksɪn], [pˈɛksɪn], [p_ˈɛ_k_s_ɪ_n] (IPA phonetic alphabet).
The ferment of rennet which coagulates the casein of milk, rennin.
A practical medical dictionary. By Stedman, Thomas Lathrop. Published 1920.