Final answer:
The dark foul-smelling lochia is the most concerning finding that requires immediate attention postpartum.
Step-by-step explanation:
Based on the information provided, the most concerning finding that requires immediate attention is the dark foul-smelling lochia. Dark foul-smelling lochia may indicate an infection such as endometritis, which is inflammation of the uterine lining. This can be a serious condition that requires prompt medical intervention to prevent further complications.