**Core Concept**
Secondary tumours in the orbit can arise from various primary sites through haematogenous, direct extension, or lymphatic routes. The orbit's rich vascular supply and proximity to cranial structures make it susceptible to metastasis from distant primary tumours.

**Why the Correct Answer is Right**
The correct answer will be the option that least likely spreads to the orbit via a common route. The orbit can be reached by metastatic cells through the superior and inferior ophthalmic veins, which connect the orbit to the facial and cerebral veins. This venous drainage facilitates the spread of tumour cells from distant sites like the breast, lung, and prostate.

**Why Each Wrong Option is Incorrect**
**Option A:** Direct extension from a primary tumour in the paranasal sinuses is a common route of spread to the orbit. This is often seen in cases of squamous cell carcinoma of the maxillary sinus. Therefore, this is a plausible option for spread.
**Option B:** Lymphatic spread from a primary site like the thyroid gland or lacrimal gland is less common but possible. The orbit's lymphatic drainage is sparse, but it can be a route for metastasis in some cases. This option might be a less likely but still possible choice.
**Option C:** Haematogenous spread from a primary tumour in the breast or prostate is a well-documented route to the orbit. The rich vascular supply and venous drainage of the orbit facilitate the seeding of metastatic cells from distant sites. This option is a likely route for spread.

**Clinical Pearl / High-Yield Fact**
When evaluating patients with orbital metastases, it's essential to consider the primary sites that commonly spread to the orbit, such as the breast, lung, and prostate. A thorough history and physical examination, along with imaging studies, can help identify the primary site and guide management.

**Correct Answer: D.**