Step 1: This is an odd-one-out question, so find the modality that is NOT part of the standard workup for a Mullerian anomaly. Step 2: The accepted methods each visualise either the cavity or the fundal outline: HSG for the cavity and tubes, ultrasound for cavity plus contour, and MRI as the most accurate map of internal and external uterine shape. Step 3: Now test CT. It carries ionising radiation and gives weak soft-tissue discrimination in the pelvis, so it cannot reliably distinguish septate from bicornuate uteri and is not used as a guided HSG technique. Step 4: Every option except CT guided HSG is a real tool, so the exception is CT guided HSG. $CT = poor\ pelvic\ soft\ tissue + radiation$ \[\boxed{\text{CT guided HSG}}\]