Source / episode info

• **Episode:**527
• **Title:**Divine Intervention Episode 527: USMLE Step 2/3 Rapid Review Series 113
• **Published:**2024-04-02
• Source:Episode page

One-liner

This episode provides a rapid review of high-yield topics including the clinical and serological hallmarks of SLE (autoimmune cytopenia, immune complex deposition), the diagnostic traps for Antiphospholipid Syndrome (high PTT not correcting with mixing study), and the key differentiating features between Osteoarthritis and Rheumatoid Arthritis on physical exam and imaging.

High-yield summary

• SLE Pathophysiology: Lupus is characterized by multi-system involvement driven by immune complex deposition, leading to inflammation in various serosal surfaces (pleuritis, pericarditis) and organs (nephritis).
• Lupus Serology: Anti-nuclear antibodies (ANA) are highly sensitive but not specific for SLE. Anti-double-stranded DNA (anti-dsDNA) and anti-Smith antibodies are highly specific markers for lupus and have a high positive predictive value.
• Autoimmune Cytopenia: Lupus commonly causes autoimmune cytopenias, including Autoimmune Hemolytic Anemia (AIHA) (autoantibodies against RBCs) and Immune Thrombocytopenia (ITP) (autoantibodies against platelets).
• Osteoarthritis (OA) Imaging Triad: Key radiographic findings include asymmetric joint space narrowing, osteophyte formation (bone overhangs), and subchondral sclerosis. Loss of articular cartilage is central to the pathology.
• Antiphospholipid Syndrome (APS) Lab Trap: Patients with APS present with a prolonged Partial Thromboplastin Time (PTT) that fails to correct following a mixing study, due to autoantibodies against clotting factors.

Learning objectives

• Differentiate the clinical, serological, and imaging findings of SLE from other connective tissue diseases.
• Recognize the unique laboratory pattern associated with Antiphospholipid Syndrome (APS).
• Distinguish between Osteoarthritis (OA) and Rheumatoid Arthritis (RA) based on physical exam, joint symmetry, and radiographic signs.
• Understand the principles of loss of complement regulatory proteins CD55/CD59 -> complement-mediated intravascular hemolysis during an active lupus flare.
• Interpret the clinical significance of different types of Coombs tests in autoimmune cytopenias.

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