A medical assembly includes a cannula and a scaling cap releasably coupled to the cannula. The cap includes a body and a sealing member integrally molded with the body to form a fluid-tight seal between the cap and cannula. The cap includes a member defining an opening for passage of a medical instrument therethrough in a fluid-tight manner. The member includes a first portion surrounding the opening and being thickened to limit tearing of the first portion, and a second portion surrounding the first portion being tapered down in thickness toward the first portion to increase flexibility of the member. The assembly includes a shaft receivable in a lumen defined by an inner surface of the cannula. The shaft includes a protrusion and the inner surface further defines a protrusion receiving formation.