Maple syrup urine disease (MSUD) is a life-threatening metabolic disorder for which randomized trials are infeasible. We present the MSUD Age-matched Standard Treatment Cohort (MATCH), a prospective natural history study of 11 infants with classic MSUD followed from neonatal diagnosis to liver transplantation. Aligned with Food and Drug Administration (FDA) guidance and International Council for Harmonisation (ICH) E9(R1), MATCH applies prespecified eligibility criteria, fixed visit cadence, adjudicated outcomes, and explicit handling of intercurrent events. Three of six outcome measures—proportional intact protein equivalent (PIPE), crisis management days (CMDs), and blood alloisoleucine concentration—are prespecified as estimands. Monte Carlo simulations show that a single-arm trial comparing 11 treated participants to MATCH controls achieves ≥90% power (p ≤ 0.025) to detect 64% fewer CMDs (3.0% vs. 8.4%), a 57% increase in PIPE (19.3% vs. 12.3%), and a 36% reduction in alloisoleucine (117 vs. 183 μM). MATCH demonstrates how protocolized natural history data serve as regulatory-grade external controls for single-arm trials.
