2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/113312We solve the problem of coherent Mikheyev-Smirnov-Wolfenstein (MSW) resonant active-to-sterile neutrino flavor conversion driven by an initial lepton number in the early universe. We find incomplete destruction of lepton number in this process and a sterile neutrino energy distribution with a distinctive cusp and high energy tail. These features imply alteration of the non-zero lepton number primordial nucleosynthesis paradigm when there exist sterile neutrinos with rest masses ~ 1 eV. This could result in better light element probes of (constraints on) these particles.4 pages, 3 figures, matches version printed in PRLAstrophysicsHigh Energy Physics - Phenomenologytext