**Core Concept**
Swan neck deformity is a type of finger deformity characterized by hyperextension at the proximal interphalangeal (PIP) joint and flexion at the distal interphalangeal (DIP) joint. This deformity often results from a disruption in the balance of the extensor and flexor tendons in the finger.

**Why the Correct Answer is Right**
The correct answer, **D. Extention at PIP and Flexion at DIP joint**, accurately describes the characteristic deformity of swan neck deformity. This occurs due to the imbalance of the extensor and flexor tendons, resulting in hyperextension of the PIP joint and flexion of the DIP joint. The extensor tendon becomes excessively tight, causing the PIP joint to hyperextend, while the flexor tendon becomes loose, allowing the DIP joint to flex. This imbalance can be caused by various factors, including trauma, rheumatoid arthritis, or other conditions affecting the tendons.

**Why Each Wrong Option is Incorrect**
* **Option A:** Flexion at PIP and DIP joint - This option describes a flexion deformity, such as that seen in boutonniere deformity, not swan neck deformity.
* **Option B:** Extension at PIP and DIP joint - This option is incorrect because it describes a straight finger, which is not characteristic of swan neck deformity.
* **Option C:** Flexion at PIP and Extension at DIP joint - This option is incorrect because it describes a boutonniere deformity, not swan neck deformity.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that swan neck deformity can be a sign of underlying tendon imbalances or conditions, such as rheumatoid arthritis, which can lead to joint instability and deformity.

**✓ Correct Answer: D. Extention at PIP and Flexion at DIP joint**