Diagnosis: Systemic Lupus Erythematosus (SLE)
The most likely diagnosis is SLE (Option B), based on the positive ANA at 1:320, pancytopenia, pleuritic chest pain with pulmonary embolism, and joint involvement—all characteristic features of lupus, while the ferritin level of 900 ng/mL is too low and the absence of fever/rash argues strongly against Adult-Onset Still's Disease. 1
Key Diagnostic Reasoning
Why SLE is the Correct Answer
ANA 1:320 strongly supports SLE: This titer exceeds the diagnostic threshold and provides 95.8% sensitivity and 86.2% specificity for systemic autoimmune rheumatic diseases when combined with clinical features 1
Pancytopenia is characteristic of SLE: Cytopenias including anemia, leukopenia, and thrombocytopenia are hallmark features of lupus 1
Pleuritic chest pain with PE fits SLE: Serositis (pleuritis) is a classic SLE manifestation, and lupus patients have increased thrombotic risk due to antiphospholipid antibodies 1
Joint pain and synovitis occur in SLE: Arthralgia and non-erosive arthritis are common lupus manifestations 1
Why NOT Adult-Onset Still's Disease (AOSD)
Ferritin level is far too low: AOSD typically presents with ferritin >1000 ng/mL, often ranging from 4000-30,000 ng/mL, with extreme levels up to 250,000 ng/mL reported 2. The ferritin of 900 ng/mL argues strongly against AOSD 1
No characteristic rash: AOSD requires an evanescent salmon-pink rash that coincides with fever spikes, typically on the trunk—this patient has no rash 2
No fever pattern described: AOSD diagnosis requires spiking fever ≥39°C for at least 7 days 2. The absence of documented fever makes AOSD unlikely
ANA is typically negative in AOSD: While not absolute, ANA and rheumatoid factor are usually absent in AOSD 2
Pancytopenia in AOSD suggests MAS: When pancytopenia occurs in Still's disease, it should alert physicians to macrophage activation syndrome, a severe complication requiring prompt immunosuppressive treatment 2
Why NOT Rheumatoid Arthritis (RA)
Systemic features too prominent: RA does not typically present with pleuritic chest pain, PE, pancytopenia, and markedly elevated ferritin 1
ANA positivity at this titer is uncommon in RA: While low-titer ANA can occur in RA, 1:320 suggests a different systemic autoimmune disease 1
Why NOT Infective Endocarditis
Normal cardiac exam: The question explicitly states normal cardiac examination, making endocarditis unlikely 1
Clinical pattern fits autoimmune disease: The constellation of positive ANA, pancytopenia, and serositis points to autoimmune rather than infectious etiology 1
Essential Next Steps for Confirmation
Immediate Testing Required
Anti-dsDNA antibodies: Highly specific for SLE using Crithidia luciliae immunofluorescence test 1
Anti-Smith (Sm) antibodies: Highly specific for SLE with strong confirmatory power 1
Urinalysis with microscopy: Essential to evaluate for lupus nephritis (proteinuria, cellular casts) 1
Complete autoantibody panel: Including anti-Ro, anti-La, and antiphospholipid antibodies to assess for thrombotic risk explaining the PE 1
Complement levels (C3, C4): Typically low in active SLE 1
Critical Clinical Pitfall
Do not be misled by elevated ferritin alone into diagnosing AOSD. The ferritin level of 900 ng/mL, while elevated, falls far below the typical AOSD range and is more consistent with general inflammatory disease 2, 1. The absence of the pathognomonic fever pattern and salmon-pink rash, combined with positive ANA and pancytopenia, makes SLE the clear diagnosis 2, 1.