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: Unknown
All of the following are direct DNA interacting cytotoxic agents except 
Select and explain. 
A. Melphalan, B. Hydroxyurea, C. Carmustine, D. Ifosfamide
B: Hydroxyurea
Answer: B: Hydroxyurea 
Explanation: All other agents are alkylating agents , whereas hydroxyurea is an anti metabolite acts at S phase of cell cycle indirectly affect DNA molecules.