A protective sheath assembly for use with a medical instrument is disclosed. The assembly comprises an elongatable hollow sheath attached to a transparent distal end plate. A tube extends from the end plate and is surrounded by the sheath. In order to provide for suctioning or irrigation of bodily cavities, a biopsy valve or other instrument is accommodated in the tube. An air or water channel may be provided for passing cleaning media over the transparent end plate, and nozzles can be embedded in the end plate to direct the cleaning media onto the end plate.