You are answering a USMLE-style medical question. Question: A 3-year-old boy is brought to the pediatrician because of abdominal pain and constipation for 3 weeks. His mother says he has been increasingly irritable recently. His vocabulary consists of 50 words and he does not use sentences. Physical examination shows pale conjunctivae and abdominal tenderness. He refers to himself by name but is unable to name body parts or count to three. Peripheral blood smear shows small, pale red blood cells with basophilic stippling. Which of the following processes is most likely impaired in this patient? Options: A) Conversion of ferrous iron to ferric iron B) Conversion of porphobilinogen to hydroxymethylbilane C) Conversion of aminolevulinic acid to porphobilinogen D) Conversion of uroporphyrinogen III to coproporphyrinogen III Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.