**Core Concept**
The clinical presentation of a child with joint swelling and pain, preceded by an upper respiratory infection, and accompanied by a petechial rash and bleeding per rectum, suggests a diagnosis related to a systemic vasculitis or an autoimmune disorder.

**Why the Correct Answer is Right**
The boy's symptoms are consistent with Henoch-Schönlein Purpura (HSP), a form of vasculitis that affects small blood vessels. HSP is characterized by the deposition of IgA immune complexes in the walls of small vessels, leading to inflammation and damage. The petechial rash, joint pain, and abdominal symptoms are classic manifestations of HSP. The presence of an upper respiratory infection can trigger the onset of HSP in susceptible individuals.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because Kawasaki disease typically presents with fever, conjunctivitis, and lymphadenopathy, but not a petechial rash or bleeding per rectum.
* **Option B:** This option is incorrect because Juvenile Idiopathic Arthritis (JIA) can cause joint pain and swelling, but the presence of a petechial rash and bleeding per rectum is not typical of JIA.
* **Option C:** This option is incorrect because Acute Rheumatic Fever (ARF) can follow a streptococcal infection, but the presence of a petechial rash and bleeding per rectum is not characteristic of ARF.

**Clinical Pearl / High-Yield Fact**
Henoch-Schönlein Purpura (HSP) is a classic example of a systemic vasculitis that can involve multiple organs, including the kidneys, gastrointestinal tract, and joints. The presence of a petechial rash and abdominal symptoms, such as bleeding per rectum, should raise suspicion for HSP in children with joint pain and a history of upper respiratory infection.

**Correct Answer:** C.