Okay, let's tackle this question. The user wants me to explain a multiple-choice question about a head injury and the characteristic hematoma shape seen on a CT scan. The correct answer is supposed to be epidural hematoma.

First, I need to recall the core concept here. The question is about differentiating types of hematomas based on their CT appearance. The key terms are biconvex and lenticular shape. I remember that epidural hematomas are typically lens-shaped because they're between the dura and the skull, which are separated by the dural sinuses. The shape is due to the bone's contour.

Now, why is epidural correct? The biconvex shape is because the hematoma is trapped between the dura and the skull. The dura is tightly adhered to the skull in some areas, so the blood can't spread much, forming a lens shape. Also, epidural hematomas are usually arterial, often from the middle meningeal artery, which is why they can expand quickly and cause rapid neurological decline.

The other options are subdural, subarachnoid, and intracerebral. Subdural hematomas are more crescent-shaped because they're under the dura and can spread along the brain's surface. Subarachnoid would show blood in the sulci and cisterns, not a biconvex shape. Intracerebral is within the brain tissue, so the shape would depend on the location and size, not the biconvex form.

Clinical pearl: Remember that the shape of the hematoma on CT helps in diagnosis. Biconvex (lens) is epidural; crescent is subdural. Also, epidural hematomas are often associated with a "lucid interval" where the patient seems okay after injury but then deteriorates. That's a key point for exams.

I need to make sure the explanation is concise but covers all the key points. Check the formatting rules: bold sections, avoid markdown for headings. Keep each section within the character limit. Make sure to mention the correct answer at the end.

**Core Concept**
The biconvex, lenticular shape of a hematoma on CT scan is a hallmark of **epidural hematoma**, caused by arterial bleeding between the dura mater and skull. This shape reflects the hematoma's containment by the dural sinuses and the rigid skull.

**Why the Correct Answer is Right**
Epidural hematomas typically arise from **middle meningeal artery** injury, a dural branch. The hematoma forms a lens shape due to pressure from the skull (convex outer margin) and the dura (convex inner margin). It is **biconvex** because it is confined by the bony calvarium and does not cross sutures. Rapid expansion can compress the brain, leading to a "lucid interval" before neurological decline.

**Why Each Wrong Option is Incorrect**
**Option A:** Subdural hematoma appears **crescent-shaped** (concave outer margin), as it forms under the dura along the brain's surface.
**Option B:** Subarachnoid hemorrhage shows **blood in sulci and cisterns** on CT, not a biconvex mass.
**Option C

✓ Correct Answer: A. Extradural haemorrhage