A 35-year-old woman from rural Maharashtra presents with a 6-month history of symmetrical peripheral neuropathy affecting the lower limbs, with stocking-glove distribution of sensory loss. On examination, she has a positive Romberg sign and loss of vibration sense. Her diet is predominantly rice-based with minimal animal protein. Which clinical feature best distinguishes her likely thiamine (vitamin B₁) deficiency from niacin (vitamin B₃) deficiency?