A Study to Evaluate the Efficacy and Safety of Mitapivat in Pediatric Participants With Pyruvate Kinase Deficiency (PKD) Who Are Regularly Transfused, Followed by a 5-Year Extension Period

Inclusion Criteria: * Written informed consent from the participant, or the participant's legally authorized representative, parent(s), or legal guardian, and the participant's assent, where applicable (informed consent/assent) must be obtained before any study-related procedures are conducted, and participants must be willing to comply with all study procedures for the duration of the study; * Aged 1 to \<18 years. Participants between 12 and 24 months of age must weigh a minimum of 7 kilograms (kg); * Clinical laboratory confirmation of pyruvate kinase deficiency (PKD), defined as documented presence of at least 2 mutant alleles in the pyruvate kinase L/R (PKLR) gene, of which at least 1 is a missense mutation, as determined per the genotyping performed by the study central genotyping laboratory; * Six to 26 transfusion episodes in the 52-week period before providing informed consent/assent; * Have complete records of transfusion history for the 52 weeks before providing informed consent/assent, defined as having all the following available: (1) all the transfusion dates, (2) the RBC transfusion volume (milliliters and/or number of units) for all the transfusions, and (3) hemoglobin concentrations within 1 week before transfusion for at least 80% of the transfusions; * Receiving folic acid supplementation as part of routine clinical care for at least 21 days before administration of the first dose of study drug, to be continued during study participation; * Female particip…