Described herein is an apparatus for guiding an elongated flexible instrument, the apparatus comprising a telescoping support assembly including a plurality of support members extending along a longitudinal axis between a proximal end and a distal end. Each of the support members comprises a ring having an inner surface, an outer surface, and at least one flange. Each flange extends from the inner surface of the ring to define a central passageway extending along the longitudinal axis and having a substantially constant width, which is configured to receive the elongated flexible instrument. The assembly is configured to selectively transition from a compressed to an expanded configuration along the longitudinal axis while each support member is interlocked with at least one other support member, and is adapted to support the elongated flexible instrument as the instrument is advanced along the longitudinal axis.