You are seeing a 23-year-old female who has presented to the acute medical assessment unit with a headache. Upon questioning, this patient informs you that the headache came on gradually over a period of 12 hours and has been present for the last 24 hours, she feels nauseous and has a sensitivity to light. The patient also informs you that the pain is on the left side of her head, is throbbing and made worse by climbing the stairs. What is the most likely diagnosis?