Strategy for the except question: tag each option as absolute or relative, then the lone relative one is the answer.
Pregnancy is an absolute no: an IUD must never be placed in a pregnant uterus. Active pelvic inflammatory disease is absolute because the device would worsen and spread infection. Undiagnosed vaginal bleeding is absolute since a serious cause such as malignancy must be excluded first.
Uterine malformation, however, sits in the relative column. A congenitally distorted or fibroid-bearing cavity may not hold the IUD properly, so insertion is discouraged, but it is not the categorical bar that pregnancy and active infection are. That makes uterine malformation the exception. Ref: Park PSM; standard OBG contraception texts.