A 20-year-old male patient developed radiating pain from the right buttock to the posterior thigh and lateral calf after cold exposure, with paroxysmal exacerbation. Physical examination: right Lasegue's sign (+), tenderness at the buttock point, popliteal point, and lateral malleolus (+), reduced pain sensation in the lateral calf and dorsum of the foot, and diminished ankle reflex. The most likely diagnosis is: