**Core Concept**
Opisthotonus is a type of abnormal posturing characterized by **hyperextension** and **arched** positioning of the body. It is often associated with **neurological disorders** or **brain injuries**. This condition involves the **central nervous system**, particularly the **brainstem**.

**Why the Correct Answer is Right**
Since the correct answer is not provided, I will explain the general mechanism. Opisthotonus is caused by an imbalance in **muscle tone**, where the **extensor muscles** overpower the **flexor muscles**. This can occur due to **damage to the brainstem** or **cerebellum**, leading to **loss of inhibition** on the extensor muscles.

**Why Each Wrong Option is Incorrect**
**Option A:** Without the correct options, it's challenging to provide specific reasons. However, opisthotonus is not typically associated with **musculoskeletal disorders**.
**Option B:** Similarly, without the correct options, it's difficult to provide a specific reason. However, opisthotonus is not usually seen in **metabolic disorders**.
**Option C:** Again, without the correct options, it's hard to provide a specific reason. However, opisthotonus is not typically associated with **cardiovascular diseases**.
**Option D:** Without the correct options, it's challenging to provide a specific reason. However, opisthotonus can be seen in **tetanus**, which is a **neurological disorder**.

**Clinical Pearl / High-Yield Fact**
Opisthotonus is a critical sign of **increased intracranial pressure** or **meningitis**, particularly in children. It is essential to recognize this condition promptly to initiate appropriate treatment.

**Correct Answer:** D. Tetanus