(Note that this list still lacks many important ships.)
(The HMS Pinafore was a fictional creation of Gilbert and Sullivan.)
... Arachnoid cysts[?] Arachnoiditis[?] Arakawa'sa syndrome II[?] Arbovirosis[?] Arc syndrome[?] Aredyld syndrome[?] AREDYLD[?] Arginase deficiency[?] ...