A 16-year-old girl presents with abdominal pain and a pelvic mass. Imaging shows a large ovarian tumor. Serum AFP is markedly elevated (>1000 ng/mL), while β-hCG is normal. Histology reveals glandular structures with enteric differentiation. Which feature best distinguishes this tumor from dysgerminoma?

## Discriminating Feature: Tumor Marker Profile ### Yolk Sac Tumor (Endodermal Sinus Tumor) vs. Dysgerminoma | Feature | Yolk Sac Tumor | Dysgerminoma | |---------|---|---| | **AFP elevation** | Marked (>1000 ng/mL common) | Absent | | **β-hCG elevation** | Absent or minimal | Absent | | **Age of presentation** | Adolescents/young adults (median 16–20 years) | Adolescents/young adults (median 15–20 years) | | **Histology** | Glandular, enteric, microcystic patterns; Schiller-Duval bodies | Uniform sheets of large clear cells; resembles seminoma | | **Bilaterality** | Rare (<5%) | Common (10–15%) | | **Chemosensitivity** | Excellent (>95% 5-year survival with BEP) | Excellent (>95% 5-year survival with BEP) | | **Radiosensitivity** | Moderate | Excellent | **Key Point:** Yolk sac tumors produce **AFP exclusively** — this is the single most reliable discriminator. Dysgerminomas do not produce significant tumor markers; they are pure germ cell tumors without endodermal differentiation. **Clinical Pearl:** In a young girl with an ovarian mass and **markedly elevated AFP**, yolk sac tumor is the leading diagnosis. The presence of enteric (endodermal) differentiation on histology confirms the diagnosis and explains the AFP production. **High-Yield:** Tumor marker patterns are the fastest way to distinguish germ cell tumors: - **Dysgerminoma** → no markers (or trace hCG if mixed) - **Yolk sac tumor** → AFP ↑↑↑ - **Choriocarcinoma** → β-hCG ↑↑↑ - **Embryonal carcinoma** → AFP + β-hCG **Warning:** Both tumors have excellent prognosis with modern chemotherapy and similar bilaterality patterns are NOT reliable discriminators. Radiosensitivity is similar in practice (both respond to chemotherapy first-line).