This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Blood, Subject name: Pathology
A 6-year-old boy presents with acute abdomen and nonhealing ulcer on medial malleolus. Peripheral smear is shown. The most likely cause of this condition is? 
Select and explain. 
A. Trinucleotide repeat, B. Antibody to RBC membrane, C. Substitution of single amino acid base, D. Genomic imprinting
C: Substitution of single amino acid base
Answer: C: Substitution of single amino acid base 
Explanation: Ref: Robbins, 8th ed. pg. 645-647Point mutation in 6th codon of b-globin that leads to replacement of glutamate by valine residue.