A newborn boy was brought into a hospital on the second day after the onset of the disease. His parents complain of his fussiness, regurgitation, fever of 38.5 ° C, red skin with infiltration in the area of his lower back. Objectively, the child is in a severe condition, inert, pale, suckles poorly. In the lumbar region, on the sacrum, and on the buttocks there is an infiltration with hyperemic areas and a soft spot 8x7 cm in its center. The stool is 10 times per 24 hours with green coloring and mucus. What is the most likely diagnosis in this case?