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
Which of the following disorder is classified under defects in metabolic pathways? 
Select and explain. 
A. Cherubism, B. Robinow syndrome, C. Severe infantile osteopetrosis, D. Pycnodysostosis
C: Severe infantile osteopetrosis
Answer: C: Severe infantile osteopetrosis 
Explanation: None