A 32-year-old woman from Mumbai is brought to the emergency department by her family with acute onset of bizarre behaviour. For the past 3 days, she has been speaking in a disorganized manner, laughing at inappropriate times, and refusing to eat or bathe. She sits motionless for hours with her arms held in unusual postures that she maintains even when moved by others. Her family reports no prior psychiatric history, and she denies any substance use. Urine drug screen is negative. Blood glucose is 110 mg/dL, and thyroid function tests are normal. On examination, she exhibits waxy flexibility and mutism. What is the most likely diagnosis?