In a 29-year-old Black man with a strongly positive ANA (nuclear speckled pattern), elevated anti‑RNP antibodies, mildly increased creatine kinase, normal ESR and ferritin, proximal muscle weakness and muscle wasting after a viral illness, and negative anti‑Sm and anti‑SSA/SSB, what is the most likely diagnosis?

Most Likely Diagnosis: Mixed Connective Tissue Disease (MCTD) with Inflammatory Myopathy

Based on your strongly positive anti-RNP antibodies (4.7, reference <0.9), nuclear speckled ANA pattern, proximal muscle weakness, mildly elevated CK (371, just above 366), and post-viral onset, you most likely have Mixed Connective Tissue Disease (MCTD) with an inflammatory myopathy component.

Key Diagnostic Features Supporting MCTD

Your presentation aligns precisely with anti-U1-RNP-positive myositis, which has a distinct clinical phenotype 1:

• Age and demographics: You're 29 years old; anti-RNP myositis typically presents around age 37, and 60% of patients are Black 1
• Muscle involvement: 80% of anti-RNP patients eventually develop weakness, though only 15% present with it initially 1
• Post-viral trigger: Your symptoms worsening after a cold is consistent with inflammatory myopathy onset
• Mildly elevated CK: Your CK of 371 (just above normal) fits the pattern—not the dramatic 10-fold elevations seen in necrotizing myopathy 2
• Low inflammatory markers: Your ESR of 2 is notably low, which can occur in MCTD despite active disease
• Negative anti-Sm: This helps distinguish MCTD from systemic lupus erythematosus

Critical Next Steps for Definitive Diagnosis

You need additional workup immediately:

1. Myositis-specific antibody panel including anti-Jo-1, anti-Mi-2, anti-SRP, anti-HMGCR 2, 3
2. Anti-Ro52 antibodies: 25-40% of anti-RNP patients with pericarditis or glomerulonephritis have coexisting anti-Ro52 1
3. Aldolase and LDH levels: These may be elevated even when CK is only mildly increased 2, 3
4. EMG (electromyography): Should show myopathic changes—polyphasic motor unit potentials, fibrillation potentials, and increased insertional activity 2
5. MRI of proximal muscles (thighs, shoulders): Will demonstrate muscle edema and inflammation 4
6. Muscle biopsy: Anti-RNP myositis can show necrotizing features in 4 of 7 patients (57%) 1, which would be critical to document

Why This Matters for Your Prognosis

The good news: Anti-RNP-positive myositis has favorable outcomes 1:

• No cancer-associated risk (unlike dermatomyositis)
• No deaths reported in the anti-RNP cohort during study follow-up
• Responds well to immunosuppressive therapy

The concerning features to monitor:

• Interstitial lung disease (ILD): Occurs in 45% of anti-RNP patients 1—you need baseline pulmonary function tests and high-resolution chest CT
• Cardiac involvement: Pericarditis occurs in 40% 1—get echocardiogram and consider cardiac MRI given your troponin history (9→6)
• Arthritis: Develops in 60% 1—watch for joint swelling
• Renal involvement: Glomerulonephritis in 25% 1—your normal kidney function is reassuring but needs monitoring

Immediate Treatment Recommendation

You should start corticosteroids now 2:

• Prednisone 1-2 mg/kg/day (approximately 60-120 mg daily for your weight) as initial therapy
• Add a steroid-sparing agent immediately: Methotrexate 15-25 mg weekly, azathioprine, or mycophenolate mofetil 2
  • Starting both together prevents prolonged high-dose steroid exposure
  • Anti-RNP myositis often requires extended treatment

Why Tylenol helps: Acetaminophen provides symptomatic relief but doesn't address the underlying inflammation. NSAIDs alone are insufficient for inflammatory myopathy 4.

Critical Pitfalls to Avoid

1. Don't dismiss mildly elevated CK: Your CK of 371 (just 5 U/L above normal) is significant in the context of weakness and positive anti-RNP. Not all myositis presents with dramatic CK elevations 2.

2. Don't delay muscle biopsy: If EMG and MRI are equivocal, biopsy is essential. Anti-RNP myositis can show necrotizing changes that require more aggressive treatment 1.

3. Don't ignore cardiac symptoms: Your previous troponin elevations (9→6) are concerning. Myocarditis can be life-threatening in inflammatory myopathy 4. You need cardiac monitoring.

4. Don't wait for more weakness: Early treatment prevents irreversible muscle damage and disability 2.

Differential Considerations

Why not polymyalgia rheumatica?

• You're too young (typically >50 years)
• You have true weakness, not just pain 5
• Your ESR is too low (PMR has highly elevated ESR)

Why not statin-induced myopathy?

• No statin exposure mentioned
• Positive anti-RNP antibodies
• Post-viral onset

Why not immune-mediated necrotizing myopathy (IMNM)?

• Possible, but less likely given:
  • Negative anti-HMGCR (need to confirm)
  • Only mildly elevated CK (IMNM typically >10× normal) 2
  • Anti-RNP positivity points toward MCTD

Monitoring During Treatment

Track these parameters every 2-4 weeks initially:

• CK, aldolase, ESR, CRP 4
• Manual muscle strength testing of proximal muscles
• Pulmonary function tests every 3-6 months
• Echocardiogram if any cardiac symptoms develop

You need urgent rheumatology referral 4, 6. This is not a condition to manage without specialist input given the multisystem involvement and need for immunosuppression.