Over the next decade he developed an understanding of the possible nature of inherited diseases of metabolism. He described the nature of recessive inheritance of most enzyme defects. In 1908, the core of this work was presented as the Croonian Lectures to the Royal College of Physicians, entitled Inborn Errors of Metabolism and published the following year. Garrod expanded his metabolic studies to cover cystinuria, pentosuria, and albinism. These three inborn errors, along with alkaptonuria are collectively called Garrod's tetrad. In 1923 he summarized these studies in an expanded edition of his best known work.