3beta-hydroxysteroid dehydrogenase that converts 3beta-hydroxysteroids to 3-ketosteroids, an essential step in the production of dafachronic acids from cholesterol. Catalyzes the dehydrogenation of lathosterol (5alpha-cholest-7-en-3beta-ol) to lathosterone (5alpha-cholest-7-en-3-one), a step required for maximal biosynthesis of Delta(7)-dafachronic acid (PubMed:22505847). Dafachronic acids act as ligands and bind directly to the nuclear hormone receptor (NHR) daf-12, suppressing dauer formation and inducing reproductive growth, they can also regulate C.elegans lifespan (PubMed:22505847, PubMed:22505849). lathosterol + NAD(+) = 5alpha-cholest-7-en-3-one + H(+) + NADH Steroid hormone biosynthesis; dafachronic acid biosynthesis. Strongly expressed in the hypodermis and posterior pharyngeal bulb and in a number of unidentified neurons of the head and tail. Mutants appear normal at 20 degrees Celsius but have Daf-c phenotypes (formation of dauer larvae at low population density in the presence of abundant food) at 27 degrees Celsius. Belongs to the short-chain dehydrogenases/reductases (SDR) family.