An apparatus, the apparatus comprising: (a) a handle assembly, wherein the handle assembly comprises: (i) a handle body, and (ii) an actuator comprising a first rotary member, wherein the first rotary member is rotatable relative to the handle body, wherein the first rotary member is configured to be engaged by and driven by a finger of a hand that grasps the handle body; (b) a shaft assembly extending distally from the handle assembly, wherein the shaft assembly comprises: (i) an external sheath fixed to the handle body, and (ii) an inner shaft coupled to the actuator, wherein the inner shaft is configured to slide longitudinally relative to the external sheath in response to rotation of the first rotary member relative to the handle body; and (c) an end effector configured to encompass a bodily lumen, wherein the end effector comprises: (i) a flexible member comprising a distal tip, wherein the flexible member extends distally from the inner shaft, (ii) a first coupling element fixed to the distal tip of the flexible member, and (iii) a second coupling element fixed to the external sheath, wherein the first and second coupling elements are configured to be magnetically attracted to each other, wherein the first and second coupling elements are biased toward each other such that the flexible member defines an adjustable loop, wherein the actuator is operable to adjust the size of the loop in response to movement of the first rotary member. (a) a handle assembly, wherein the handle assembly comprises: (i) a handle body, and (ii) an actuator comprising a first rotary member, wherein the first rotary member is rotatable relative to the handle body, wherein the first rotary member is configured to be engaged by and driven by a finger of a hand that grasps the handle body; (i) a handle body, and (ii) an actuator comprising a first rotary member, wherein the first rotary member is rotatable relative to the handle body, wherein the first rotary member is configured to be engaged by and driven by a finger of a hand that grasps the handle body; (b) a shaft assembly extending distally from the handle assembly, wherein the shaft assembly comprises: (i) an external sheath fixed to the handle body, and (ii) an inner shaft coupled to the actuator, wherein the inner shaft is configured to slide longitudinally relative to the external sheath in response to rotation of the first rotary member relative to the handle body; and (i) an external sheath fixed to the handle body, and (ii) an inner shaft coupled to the actuator, wherein the inner shaft is configured to slide longitudinally relative to the external sheath in response to rotation of the first rotary member relative to the handle body; and (c) an end effector configured to encompass a bodily lumen, wherein the end effector comprises: (i) a flexible member comprising a distal tip, wherein the flexible member extends distally from the inner shaft, (ii) a first coupling element fixed to the distal tip of the flexible member, and (iii) a second coupling element fixed to the external sheath, wherein the first and second coupling elements are configured to be magnetically attracted to each other, wherein the first and second coupling elements are biased toward each other such that the flexible member defines an adjustable loop, wherein the actuator is operable to adjust the size of the loop in response to movement of the first rotary member. (i) a flexible member comprising a distal tip, wherein the flexible member extends distally from the inner shaft, (ii) a first coupling element fixed to the distal tip of the flexible member, and (iii) a second coupling element fixed to the external sheath, wherein the first and second coupling elements are configured to be magnetically attracted to each other, wherein the first and second coupling elements are biased toward each other such that the flexible member defines an adjustable loop, wherein the actuator is operable to adjust the size of the loop in response to movement of the first rotary member.