Name the largest and most populous city in Bihar after Patna.

The largest and most populous city in Bihar after Patna is Gaya. Gaya is a historically significant city known for its religious importance, especially for Buddhists and Hindus. It attracts pilgrims from all over the world due to the presence of the Mahabodhi Temple, a UNESCO World Heritage Site, where Lord Buddha is said to have attained enlightenment. Additionally, Gaya is an important center for trade, commerce, and tourism in the region.