Okay, let's tackle this question. The patient had a brawl and now can't extend his distal interphalangeal (DIP) joint. The X-ray is normal, so no fracture. The key here is figuring out the cause of the inability to extend. The DIP joint is controlled by the flexor digitorum profundus (FDP) muscle, but wait, FDP actually causes flexion. Wait, no, maybe I'm mixing things up. Wait, the extensor mechanism for the fingers is more complex. The extensor digitorum tendon extends the DIP joint. If there's an injury to the extensor tendon here, especially at the insertion point, that could cause the problem.

Wait, but the question says inability to extend. So if the extensor tendon is damaged, the patient can't extend. The FDP is for flexion, so if that's injured, the DIP would be hyperextended, not flexed. Wait, maybe the patient has a mallet finger. Mallet finger is when the extensor tendon at the DIP is ruptured, often from a forceful flexion injury. So the DIP can't extend, and the finger is held in flexion. That fits with the scenario here. Since the X-ray is normal, it's a tendon injury, not a fracture. So the next step would be splinting the DIP in extension to let the tendon heal.

Looking at the options, the correct answer would involve splinting the DIP joint in extension. The other options might be splinting the PIP joint, which is for different injuries. For example, if the PIP is injured, you might splint that. But for mallet finger, it's the DIP. So the next step is to splint the DIP in extension. Let me check the options again. The correct answer is probably option C or D, depending on the options given. But since the user hasn't provided the options, I'll proceed with the explanation based on the scenario.

So, the core concept is mallet finger due to extensor tendon rupture. The correct management is splinting in extension. The clinical pearl is to recognize mallet finger as a tendon injury, not a fracture, and treat with splinting.

**Core Concept**
The inability to extend the distal interphalangeal (DIP) joint after trauma suggests **mallet finger**, a rupture of the **extensor digitorum tendon** at the DIP joint. This injury results from forced flexion of the DIP during trauma, causing avulsion of the tendon or avulsion fracture (not seen on plain X-ray if the avulsion is small). Diagnosis relies on clinical examination and functional testing.

**Why the Correct Answer is Right**
Mallet finger is managed conservatively with **splinting the DIP joint in full extension** for 4–6 weeks to allow tendon healing. Splinting prevents flexion, avoiding further damage and promoting reattachment of the tendon to its insertion. If splinting fails or in cases of large avulsion fractures, surgical repair (e.g., K-wire fixation) may be required. The absence of fracture on X-ray confirms this is a **tendon injury**, not a bony avulsion requiring immediate surgery.

**Why Each Wrong Option is Incorrect**
**Option A

✓ Correct Answer: A. Splint