A 58-year-old man from rural India presents with gradual, painless vision loss over 2 years. He works as a farmer and spends 8–10 hours daily in bright sunlight without protective eyewear. On slit-lamp examination, the cataract begins at the periphery of the lens and progresses centripetally, with a characteristic brown discoloration at the nucleus. Visual acuity is 6/36 in both eyes. Which type of cataract is most likely responsible for his presentation?