"A Puro Dolor" is a song recorded by Puerto Rican band Son by Four. It was written by Omar Alfanno and released as the first single of the second studio album of the band in 2000. Two versions of the track were produced for the album; one as a salsa and the other as a ballad. The ballad version was arranged by Alejandro Jaén.

The song reached number-one on Billboard Top Latin Songs chart, and became the longest running chart topper of its history, spending 20 weeks at the top; this record was broken five years later by Colombian singer-songwriter Shakira with "La Tortura" which spent 25 weeks at number-one. "A Puro Dolor" also reached the Billboard 100; this led to the recording of an English-language version of the track "Purest of Pain", which was also charted in the United States.

The composition also met with critical acclaim receiving the Lo Nuestro Award, the Billboard Music Award and a Latin Grammy Award nomination. In 2009, "A Puro Dolor" was named the best-performing Latin single of the 2000s decade in the United States. Two music videos were made for the ballad and the English versions. The record was covered by Mexican group Dinora y la Juventud and Brazilian boy band KLB both of which received radio airplay.

I wrote it in ten minutes, under deadline, with a bottle of water by my side.

In 1998, Son by Four released their first album, Prepárense. The success of the singles "Nada" and "No Hay Razón", distributed in Puerto Rico by the independent label RJO, captured the attention of the Sony Music CEO Oscar Llord, who made an agreement with RJO to re-release the album and start the recording sessions of the next album. The first single taken from the following album was "A Puro Dolor", written and arranged by Panamanian record producer Omar Alfanno, with a ballad version arranged by Alejandro Jaén. Alfanno, well known for his work with salsa recording singers Marc Anthony, Víctor Manuelle, Jerry Rivera and Gilberto Santa Rosa was in charge of the album production. Alfanno declared that he wrote the song in ten minutes, and that after the success of the track, he analyzed the composition, since he did not understand how "A Puro Dolor" became that popular. He named it "the Cinderella of all his songs."

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