Five-year-old Amy wakes her parents up at 3 am crying and complaining of a sore neck, severe headache, and feeling sick to her stomach. She has a temperature of 40°C (104°F) and hides her eyes, saying that the lights are too bright. The emergency physician suspects meningitis and performs a lumbar tap. Using your knowledge of neuroanatomy, explain into which space and at what level of the vertebral column the needle will be inserted to perform this test. Which fluid is being obtained and why?