This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pathology
The heme portion of the hemoglobin molecule consists of: 
Select and explain. 
A. Porphyrin ring with a molecule of Fe in the center, B. A polypeptide chain containing Fe, C. A pyrole ring with four molecules of Fe in the center, D. Four porphyrin rings, each containing a molecule of Fe in the center
D: Four porphyrin rings, each containing a molecule of Fe in the center
Answer: D: Four porphyrin rings, each containing a molecule of Fe in the center 
Explanation: Structurally, porphyrin consists of four pyrrole rings (five-membered closed structures containing one nitrogen and four carbon atoms) linked to each other by methine groups (−CH=).
The iron atom is kept in the centre of the porphyrin ring by interaction with the four nitrogen atoms.