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: Pharmacology
Hydroxyurea mechanism of action in cancer is by inhibiting the enzyme ? 
Select and explain. 
A. Ribonucleotide diphosphate reductase, B. Ribonucleotide oxidase, C. DNA lyase, D. DNA synthetase
A: Ribonucleotide diphosphate reductase
Answer: A: Ribonucleotide diphosphate reductase 
Explanation: Ans. is 'a' i.e., Ribonucleoside diphosphate reductase Hydroxyurea It blocks the conversion of ribonucleotides to deoxyribonucleotides by inhibiting the enzyme ribonucleoside diphosphate reductase; thus inhibits the DNA synthesis; S phase specific. Myelosuppression is the major toxicity. GI disturbances and cutaneous reactions (pigmentation) also occur. It is used in CML, psoriasis, polycythemia vera and some solid tumors. It is also used as radiosensitizer before radiotherapy and is a first line drug for sickle cell disease in adults.