Final answer:
The most appropriate next step in the management of this patient is dilation and suction curettage to remove any retained products of conception and control the bleeding.
Step-by-step explanation:
The most appropriate next step in the management of this patient, a 22-year-old G1P0 woman presenting with heavy vaginal bleeding at eight weeks gestation, is C. Dilation and suction curettage. This is because she has a dilated cervical os and is experiencing brisk bleeding, indicating a possible miscarriage or incomplete abortion. Dilation and suction curettage is the most effective and expedient method to remove any retained products of conception and control the bleeding.