The adjective <em>retentive </em>means having the ability to memorize or remember facts and impressions easily.
The root of a word is the word in its basic form, without any prefixes or suffixes. The prefix <em>re-</em> in retentive means <em>back,</em> while the word roots <em>-tin-, -ten-</em> and -<em>tain</em> mean to hold.
Because anyone could’ve said they were a doctor and no one had any idea of knowing if they were telling the truth or not. Also because doctors were still learning about medicines and surgeries.