Vitamin D3 with IUPAC approved numbering for secosteroids. Since secosteroids are derived from steroids, they retain the same numbering and labels as steroids. B-ring of steroids is broken in 9,10-secosteroids like vitamin D3 and only rings A, C and D remain.
<a href="//commons.wikimedia.org/wiki/User_talk:Keministi" class="mw-redirect" title="User talk:Keministi">Keministi</a>
<span class="int-own-work" lang="en">Own work</span>