The correct answer is D.
The compound in choice D is pyrimidine itself, a heterocyclic analog
of benzene containing two nitrogen atoms, positioned as shown. Pyrimidine
and its derivatives are essential components of nucleic acids such as DNA
and RNA. Choice A is almost, but not quite, the general structural formula
for a steroid; it differs from the standard steroidal ring structure in that
the ring on the far right has six carbons instead of five. Choices B and E,
known as quinoline and pyridine respectively, each have just one nitrogen
atom and thus fail to meet the definition of a pyrimidine. Choice C is purine,
another essential component of nucleic acids.
(A)
Distortion. Molecule is a steroid-like compound
with four rings.
(B)
Distortion. Molecule is a poison known as
quinoline.
(C)
Distortion. Molecule is purine.
(E)
Distortion. Molecule is pyridine.
