Choriocarcinoma — Bilaminar Trophoblast No Villi

A 28-year-old woman from rural India presents with 6 months of irregular vaginal bleeding and lower abdominal pain. She reports a hydatidiform mole evacuation 4 months ago. Pelvic ultrasound shows a heterogeneous uterine mass with extensive hemorrhage and necrosis. Serum β-hCG is 850,000 mIU/mL. Histopathology reveals biphasic proliferation of pleomorphic cytotrophoblasts and multinucleated syncytiotrophoblasts with extensive hemorrhage and necrosis. The structure marked **D** in the diagram is notably absent. Which of the following best explains why the **absence of chorionic villi (D)** is the KEY HISTOLOGICAL FEATURE that distinguishes this lesion from a hydatidiform mole?