**Question:** All of the following statements about extramedullary tumours are true except

A. Tumours that arise from cells outside the bone marrow.
B. Extramedullary tumours can originate from all cell types.
C. Extramedullary tumours can be malignant or benign.
D. Extramedullary tumours cannot be detected using blood tests.

**Correct Answer:** **D. Extramedullary tumours cannot be detected using blood tests.**

**Core Concept:** Extramedullary tumours are neoplasms that develop from cells outside the bone marrow, which is different from intramedullary tumours that arise within the bone marrow. Tumours can originate from various cell types, and both malignant (cancerous) and benign (non-cancerous) extramedullary tumours exist. Blood tests primarily focus on analysing the composition of blood, such as haemoglobin, white blood cell count, and platelet count, which can be affected by extramedullary tumours but not necessarily detect the presence of the tumour itself.

**Why the Correct Answer is Right:** Extramedullary tumours can be detected through various diagnostic methods such as imaging studies (e.g., X-rays, CT scans, MRI) and biopsy. Blood tests alone cannot confirm the presence of an extramedullary tumour. Detecting a tumour through blood tests would be misleading as it may show changes in blood parameters due to the tumour but not directly confirm its presence.

**Why Each Wrong Option is Incorrect:**

A. This statement is true as extramedullary tumours are indeed tumours that develop from cells outside the bone marrow.
B. While extramedullary tumours can originate from various cell types, this option is incorrect because blood tests can provide information about the affected cells or tissues, but not specifically detect the presence of the tumour itself.
C. This statement is true as extramedullary tumours can be both malignant and benign.
D. This statement is incorrect because blood tests can show changes in blood parameters due to the tumour, but they cannot directly confirm the presence of an extramedullary tumour.

**Clinical Pearls:** Although blood tests may provide hints about the tumour's effects, they are not reliable for confirming the presence of an extramedullary tumour. Diagnostic imaging and biopsy are essential for making a definitive diagnosis and assessing the extent of the tumour.