A 5-year-old child has a cavity localised in the deep dentin on the masticatory surface of the 75th tooth. The cavity is filled with softened pigmented dentin and food remains. The child complains of dull pain when taking food or cold liquids. Probing of the cavity floor is slightly painful; when the dentin is removed with excavator, pink pulp is visible through the cavity floor. What is the most probable diagnosis?