**Question:** All of the following ovarian tumours usually occur bilaterally. Except -

A. Brenner's Tumour
B. Ovarian Stromal Cell Tumour
C. Germ Cell Tumour
D. Sertoli-Leydig Cell Tumour

**Core Concept:** Ovarian tumours are diverse group of lesions that can affect the ovaries, with different subtypes having various clinical implications and treatment approaches. In this question, we are focusing on tumours that are typically bilateral (occurring in both ovaries).

**Why the Correct Answer is Right:**

Correct Answer: Germ Cell Tumour (Option C)

Germ cell tumours are the most common type of ovarian cancer, accounting for around 80-90% of all ovarian malignancies. They arise from the abnormal development of germ cells (the cells that give rise to sperm and eggs). Since germ cell tumours often originate from undifferentiated cells, they can grow rapidly and have a high potential for producing hormones or other substances, leading to various clinical presentations and diagnostic challenges.

However, germ cell tumours are generally unilateral (occurring in just one ovary) and less commonly bilateral. This is why germ cell tumours are the exception among the options provided, making Option C the correct answer.

**Why Each Wrong Option is Incorrect:**

A. Brenner's Tumour (Option A): Brenner's tumour is a benign (non-cancerous) follicular cell neoplasm that primarily affects women over 50 years old. Although it can be bilateral in some cases, it is generally considered a unilateral lesion.

B. Ovarian Stromal Cell Tumour (Option D): Ovarian stromal cell tumours are generally benign and can be bilateral in some instances. However, the majority are unilateral lesions, making Option D incorrect.

C. Germ Cell Tumour (Option C, correct): As explained above, germ cell tumours are generally unilateral and rarely bilateral. This makes Option C the correct answer amongst the wrong options.

D. Sertoli-Leydig Cell Tumour (Option D): Sertoli-Leydig cell tumours are a type of gonadal stromal cell tumour. They can be bilateral in up to 40% of cases, making them less likely than other options to be bilateral.

**Clinical Pearls:**

1. Since germ cell tumours are typically unilateral, patients with bilateral ovarian tumours should raise suspicion for other diagnoses, such as Brenner's tumour or stromal cell tumours like Sertoli-Leydig cell tumour or mixed germ cell-stromal tumour.
2. Ovarian tumours are usually diagnosed in women of reproductive age, and the clinical presentation can vary depending on the tumour type and size, ranging from asymptomatic to causing pain, abdominal distension, or reproductive dysfunction.
3. Imaging studies like ultrasound, CT scan, MRI, and serum markers can be helpful in differentiating benign from malignant lesions and in determining the size and nature of the tumours.

4. Treatment options for ovarian tumours depend on the