Which Russian composer wrote Pictures at an Exhibition?

Mussorgsky, in 1874, written for solo piano in about three weeks after the death of his friend Viktor Hartmann, an artist and architect - the piece is a walk through a memorial exhibition of Hartmann's work, with a recurring Promenade theme representing the composer moving between pictures.

Most of the pictures are lost. Only a handful of the works it depicts have been identified, so the music is now the primary record of the art it was written about - a portrait that outlived its subject entirely.

It is best known in an orchestration Mussorgsky did not write. Ravel's 1922 version is the one usually performed, and it is a masterpiece of orchestration that also smooths out the original's deliberate awkwardness - Mussorgsky wrote rough, modal, non-Western-sounding music on purpose, and his colleagues kept tidying him up. Rimsky-Korsakov rewrote much of his output after his death from alcoholism at 42, and it took the twentieth century to go back to the originals and conclude that the roughness was the point.