A 38-year-old female tailor from Mumbai presents with a 6-month history of progressive weakness affecting the 4th and 5th fingers of her right hand. She reports difficulty with fine motor tasks such as threading needles. On examination, there is atrophy of the first dorsal interosseous muscle, weakness of finger abduction and adduction, and a positive Froment's sign. Notably, sensation is completely intact over the dorsal and volar surfaces of the hand, including the medial one-and-a-half fingers. Which anatomical variant or site best explains this presentation?