**Question:** Transient synovitis {toxic synovitis} of the hip is characterized by all of the following, except -

A. Inflammation of the hip joint capsule
B. Involvement of the hip joint in patients with septic arthritis
C. Painful swelling of the hip joint following trauma
D. Limited range of motion in the hip joint

**Correct Answer:** B. Involvement of the hip joint in patients with septic arthritis

**Core Concept:**
Transient synovitis, also known as toxic synovitis, is a self-limiting condition characterized by inflammation of the hip joint capsule. It commonly occurs in children between the ages of 2 and 8 years but can affect individuals of any age. The condition is typically caused by an irritant or toxin within the joint, leading to a localized inflammatory response.

**Why the Correct Answer is Right:**
The correct answer is C - "Painful swelling of the hip joint following trauma" because transient synovitis is not typically caused by trauma. Although the condition may present with pain and swelling following minor trauma, the primary cause is typically an irritant or toxin within the joint, like environmental irritants or infections, leading to the localized inflammatory response. Transient synovitis is not a direct consequence of a traumatic event.

**Why Each Wrong Option is Incorrect:**
A. **Inflammation of the hip joint capsule (Transient synovitis)**: This option is correct as it describes the hallmark feature of transient synovitis.

B. **Involvement of the hip joint in patients with septic arthritis**: Septic arthritis is a distinct entity caused by bacterial infection, whereas transient synovitis is not. Septic arthritis is a systemic infection affecting the joint, while transient synovitis is a localized inflammatory response to an irritant or toxin within the joint.

C. **Painful swelling of the hip joint following trauma**: As mentioned earlier, transient synovitis is not directly caused by trauma. The condition results from an irritant or toxin within the joint, leading to a localized inflammatory response.

D. **Limited range of motion in the hip joint**: Transient synovitis is characterized by pain, swelling, and warmth, but not necessarily limited range of motion. While some patients may experience decreased mobility due to the pain and swelling, not all cases present with limited joint mobility.

**Clinical Pearl:**
Transient synovitis should be considered in the differential diagnosis of a child with acute onset hip pain, swelling, and warmth, especially those with no history of trauma. In contrast to septic arthritis, the patient typically has no systemic symptoms, such as fever, chills, or joint tenderness. Clinical examination and imaging studies, such as ultrasound or MRI, can help differentiate between transient synovitis and septic arthritis, as the latter typically presents with joint effusion, joint space narrowing, and signs of infection on imaging studies.