**Question:** Which of the following is not seronegative spondyloarthropathy?

A. Ankylosing spondylitis (AS)
B. Reactive arthritis (ReA)
C. Psoriatic arthritis (PsA)
D. Infectious arthritis (e.g., tuberculosis)

**Correct Answer: D. Infectious arthritis (e.g., tuberculosis)**

**Core Concept:** Seronegative spondyloarthropathies are a group of related autoimmune diseases characterized by inflammation of the joints, spine, and other connective tissues. These conditions are typically associated with the absence of specific antibodies or antigens in the patient's blood.

**Why the Correct Answer is Right:** Infectious arthritis, such as tuberculosis, is not considered a seronegative spondyloarthropathy because these conditions are caused by bacterial or viral infections, rather than autoimmune mechanisms. In contrast, seronegative spondyloarthropathies like ankylosing spondylitis (AS) and psoriatic arthritis (PsA) are associated with the presence of specific antibodies or antigens, making them seropositive (not seronegative). Reactive arthritis (ReA) is a seronegative spondyloarthropathy because it is a post-infectious condition resulting from a preceding gastrointestinal, urological, or sexually transmitted infection.

**Why Each Wrong Option is Incorrect:**

A. Ankylosing spondylitis (AS) is a seropositive spondyloarthropathy caused by the presence of HLA-B27 antigen, making it a seronegative condition.

B. Reactive arthritis (ReA) is a seronegative spondyloarthropathy caused by preceding infections (e.g., enteropathic arthritis, after gastrointestinal infections; Reiter's syndrome after urethritis or conjunctivitis).

C. Psoriatic arthritis (PsA) is a seropositive condition associated with the presence of specific antibodies, making it a seronegative spondyloarthropathy.

**Clinical Pearl:** The distinction between seronegative and seropositive spondyloarthropathies is important for accurate diagnosis and appropriate management of these conditions. Seronegative spondyloarthropathies, like AS and ReA, are more likely to present in young adults, often with morning stiffness, axial (back and pelvic) pain, and enthesitis (inflammation at the site of attachment of tendons, ligaments, or aponeuroses to bone). In contrast, seropositive spondyloarthropathies, like PsA, are associated with specific antibodies or antigens, making diagnosis easier but not relevant to the current question.

**Why Each Wrong Option is Incorrect:**

A. Ankylosing spondylitis (AS) is a seropositive spondyloarthritis because it is associated with the presence of HLA-B27 antigen, which makes diagnosis easier as opposed to seronegative