A 58-year-old man from rural Maharashtra presents with a 6-month history of progressive constipation alternating with diarrhoea, abdominal pain, and a 4 kg weight loss. On examination, a palpable mass is felt in the left lower abdomen. Colonoscopy reveals a stricturing lesion at the sigmoid colon with irregular, ulcerated mucosa. Biopsy shows infiltrating glands with marked desmoplastic stromal reaction. Which histological grade would carry the worst prognosis in this case?