Oligohydramnios is almost always associated with an inability to excrete urine and is most commonly associated with bilateral renal agenesis, posterior urethral valves (in males), and sometimes placental insufficiency. Any profound oligohydramnios can cause Potter sequence. In this question, urethral obstruction as an answer option is wrong because it causes oligohydramnios (the question was asking about polyhydramnios) because it leads to decreased fetal urine output and decreased amniotic fluid production by the fetus.