Bell Palsy — Idiopathic Facial Palsy

A 6-year-old child presents with acute-onset left-sided facial weakness. On examination, the left eye cannot close completely, the left corner of the mouth droops, and the nasolabial fold on the left is flattened. Notably, the forehead wrinkles on the left side are absent (marked as **D** in the diagram). Which of the following clinical findings best explains why the structure marked **D** distinguishes this condition from an upper motor neuron facial palsy?