Step 1: Understanding the Concept:
We need to count the total number of carbon atoms in pregnane, the parent steroid skeleton behind progesterone and the corticosteroid hormones.
Step 2: Key Formula or Approach:
Build the count piece by piece: start from the basic four-ring steroid nucleus, add the angular methyl groups, then add whatever side chain sits at C17.
Step 3: Detailed Explanation:
The steroid nucleus itself, made of three six-membered rings (A, B, C) fused to one five-membered ring (D), contains 17 carbon atoms.
Two angular methyl groups are attached to this nucleus, one at C10 and one at C13, adding 2 more carbons for a running total of 19.
Pregnane additionally carries a two-carbon side chain at C17, an ethyl-type group that becomes the site of the ketone in progesterone.
Adding these 2 side chain carbons to the running total of 19 gives \[ 17 + 2 + 2 = 21 \] carbon atoms in total.
Step 4: Final Answer:
Pregnane contains 21 carbon atoms.