A robotic medical system comprises a medical instrument assembly having a retainer, a serial array of instruments disposed in the retainer, a chamber, and an instrument driver. The robotic medical system further comprises a user interface configured for generating at least one command signal, a drive unit coupled to the first mechanism, second mechanism, and instrument driver, and an electric controller configured, in response to the command signal(s), for directing the drive unit to linearly displace the array of instruments within the retainer, to displace a selected one of the instruments from the retainer into the chamber, and to distally advance the instrument driver within the chamber to engage the selected instrument.