The A5203 is an urban A-road in London.
Owing to a one-way system the A5203 has two southern termini. Northbound traffic starts on the A5200 at the northern end of King's Cross Station and heads east along Wharfdale Road to meet the other direction at Caledonian Road. Southbound traffic heads south from here along Caledonian Road to meet the A501 and A5200 Inner Ring Road one block east of King's Cross Station.
The A5203 now heads north along Caledonian Road and crosses the Regents Canal and the North London Line. It passes Pentonville Prison and Caledonian Road tube station before it splits again.
As in the south, so the A5203 has two northern termini. Northbound traffic turns left onto Hillmarton Road to end on the A503 opposite Holloway Prison. Southbound traffic starts on the A503 adjacent to the A1 Holloway Road before heading south along Caledonian Road. Coincidentally the A503 is also following a one-way loop here and so the northbound A5203 joins both sections, here a few yards apart.
As may be expected, Caledonian Road is the original route of the A5203. The extra bits for northbound traffic were added later.