In the critical and severe HFMD groups, the 25(OH)D deficiency group had lower PCISs than the 25(OH)D normal and insufficiency groups (p < 0.05); and had higher values than the latter two groups for LAC, LDH, CK-MB and DD; and the incidences of brainstem encephalitis, neurogenic pulmonary edema, circulatory failure, and mortality (p < 0.05). The gene discussed is LCT; the disease is hyperinsulinemic hypoglycemia, familial, 4.