You are a helpful assistant. When the user asks a question, your response must include two parts: first, the reasoning process enclosed in <think>...</think> tags, then the final answer enclosed in <answer>...</answer> tags.Please provide a clear, concise response within <answer> </answer> tags that directly addresses the question.
system
<image>This two-year-old boy was brought to CHP with 3-week history of occipital headaches. The mother tells you he sleeps with his neck extended. During the last week prior to presenting to CHP he vomited once/day. On neurologic exam there was no noticeable weakness, gait abnormality, balance or visual changes. The mass lesion is partially resected. Your diagnosis? <image 1> <image 2>
A. Pilocytic astrocytoma
B. Subependymoma
C. Glioblastoma
D. Anaplastic ependymoma
E. Medulloblastoma

Please answer the question with option letter from given choices in the format of Answer: <option letter>. You FIRST think about the reasoning process as an internal monologue and then provide the final answer. The reasoning process MUST BE enclosed within <think> </think> tags. Please answer the final answer within the <answer> </answer> tags.
user
C
This two-year-old boy was brought to CHP with 3-week history of occipital headaches. The mother tells you he sleeps with his neck extended. During the last week prior to presenting to CHP he vomited once/day. On neurologic exam there was no noticeable weakness, gait abnormality, balance or visual changes. The mass lesion is partially resected. Your diagnosis? <image 1> <image 2>
A. Pilocytic astrocytoma
B. Subependymoma
C. Glioblastoma
D. Anaplastic ependymoma
E. Medulloblastoma

Please answer the question with option letter from given choices in the format of Answer: <option letter>. You FIRST think about the reasoning process as an internal monologue and then provide the final answer. The reasoning process MUST BE enclosed within <think> </think> tags. Please answer the final answer within the <answer> </answer> tags.
val