**Core Concept**
The question is testing the understanding of **hip pathology** in children, specifically the presentation and clinical signs of a common condition affecting the hip joint. The **limping child** with limited mobility and tenderness in a specific area suggests a localized problem that requires precise diagnosis.

**Why the Correct Answer is Right**
Given the age, weight, and symptoms of the child, including **limitation of abduction and internal rotation**, along with **tenderness in Scarpa's triangle**, the presentation is highly suggestive of a condition where the femoral head is not properly seated in the acetabulum, leading to restricted movement. The fact that **on flexing the hip, the limb is abducted** indicates a specific mechanical issue related to the hip joint.

**Why Each Wrong Option is Incorrect**
**Option A:** Would not typically present with such specific mechanical findings related to hip movement.
**Option B:** Might present with pain but the specific pattern of limited abduction and internal rotation with relief on flexion is not characteristic.
**Option C:** Could involve the hip but the pattern of symptoms and signs, especially the relief with flexion, does not fit.
**Option D:** Is not a common diagnosis for the symptoms described.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that **developmental dysplasia of the hip (DDH)** can present in various ways, including asymmetrical skin folds, limited abduction, and sometimes a palpable "clunk" as the femoral head moves in and out of the acetabulum. However, the specific presentation here hints more towards a condition that affects the mechanical alignment and movement of the hip joint, suggesting a different but related issue.

**Correct Answer:** D. Developmental Dysplasia of the Hip.