**Core Concept**
Germ cell tumours of the ovary are a heterogeneous group of neoplasms originating from the primordial germ cells. These tumours can be classified into several subtypes, including dysgerminomas, yolk sac tumours, embryonal carcinomas, choriocarcinomas, and mature teratomas.

**Why the Correct Answer is Right**
To determine which option is not a germ cell tumour of the ovary, we need to review the characteristics of each option. Germ cell tumours typically arise from the ovary and are composed of cells that are precursors to the gametes. Dysgerminomas, yolk sac tumours, embryonal carcinomas, and choriocarcinomas are all well-established subtypes of germ cell tumours.

**Why Each Wrong Option is Incorrect**
**Option A:** Dysgerminomas are actually the most common type of germ cell tumour of the ovary.
**Option B:** Yolk sac tumours are a type of germ cell tumour that is often seen in children and young adults.
**Option C:** Embryonal carcinomas are a subtype of germ cell tumour that can be aggressive and may be associated with other tumour types, such as choriocarcinomas.
**Option D:** This option is not provided, but we can infer that the correct answer will be a tumour type that is not classified as a germ cell tumour of the ovary.

**Clinical Pearl / High-Yield Fact**
The International Federation of Gynaecology and Obstetrics (FIGO) has established a staging system for ovarian germ cell tumours, which is used to predict prognosis and guide treatment.

**Correct Answer: D.**