I have a sore throat with fever, chills, and myalgias, and a few hours later developed a pruritic, non‑papular rash—what is the most likely diagnosis and appropriate management?

Scarlet Fever (Group A Streptococcal Pharyngitis with Scarlatiniform Rash)

The most likely diagnosis is scarlet fever caused by Group A Streptococcus (GAS), and you should obtain a rapid antigen detection test (RADT) or throat culture immediately, then treat with penicillin V or amoxicillin for 10 days if positive. 1, 2

Why This Is Scarlet Fever

The clinical presentation—sudden onset sore throat with fever, chills, myalgias, followed hours later by a pruritic, non-papular (maculopapular/sandpaper-like) rash—is the classic sequence for scarlet fever. 1, 3, 4

• The temporal relationship is key: throat symptoms appear first, then the rash develops 1-2 days later (in your case, hours). 1
• The rash in scarlet fever is characteristically described as "sandpaper-like," salmon-pink, and maculopapular (not truly papular in the raised sense), often mildly pruritic, predominantly affecting the trunk and proximal limbs. 5, 3, 4
• Systemic symptoms (fever, chills, myalgias) accompanying acute pharyngitis strongly favor bacterial over viral etiology. 5, 1

Immediate Diagnostic Steps

You must obtain microbiological confirmation before treating—clinical diagnosis alone is unreliable, even with classic features. 5, 1

• Perform a rapid antigen detection test (RADT) on a pharyngeal swab; a positive result is diagnostic and warrants immediate treatment. 1, 2
• If the RADT is negative and you are a child or adolescent, a backup throat culture is mandatory because RADT sensitivity is only 79-88%, and missing GAS risks rheumatic fever. 1, 2
• In adults, backup culture after negative RADT is optional given the extremely low risk of acute rheumatic fever. 1

Do not treat empirically without testing—this leads to unnecessary antibiotic use for viral pharyngitis, which accounts for the majority of acute pharyngitis cases. 5, 1

Treatment Protocol for Confirmed GAS

First-line therapy is penicillin V or amoxicillin for a full 10-day course. 1, 2

• Penicillin remains the gold standard due to proven efficacy, narrow spectrum, safety, low cost, and zero resistance. 1, 2
• Amoxicillin is an acceptable alternative, particularly in younger children, and may be dosed once or twice daily for better compliance. 1, 2
• The 10-day duration is non-negotiable—it is required for bacterial eradication and prevention of acute rheumatic fever, glomerulonephritis, bacteremia, pneumonia, endocarditis, and meningitis. 1, 2, 3

Alternative Regimens

• For non-anaphylactic penicillin allergy: use a narrow-spectrum cephalosporin (cefadroxil or cephalexin) for 10 days. 1
• For true penicillin allergy or anaphylaxis: clindamycin (~1% GAS resistance in the U.S.) or a macrolide (5-8% resistance; use with caution). 1

Supportive Care

• Analgesics (acetaminophen or NSAIDs; avoid aspirin in children due to Reye syndrome risk). 1
• Adequate hydration, warm saline gargles, topical anesthetics, and rest. 1

Critical Differential Diagnoses to Exclude

While scarlet fever is most likely, you must briefly consider life-threatening mimics:

• Rocky Mountain Spotted Fever (RMSF): Rash starts on ankles/wrists and spreads to palms/soles with petechiae; requires immediate doxycycline without awaiting serology if any tick exposure history exists. 1, 6
• Disseminated gonococcal infection: Migratory polyarthritis with pustular skin lesions; consider in sexually active individuals. 6
• Secondary syphilis: Maculopapular rash involving palms/soles with polyarthritis. 6
• Adult-Onset Still's Disease (AOSD): Salmon-pink evanescent rash with high-spiking quotidian fever and polyarthritis; more common in adults, not children. 5, 6

Your presentation lacks the key features of these conditions (no palm/sole involvement, no tick exposure, no polyarthritis, no quotidian fever pattern), making scarlet fever the clear diagnosis. 1, 6

Common Pitfalls to Avoid

• Do not assume all pharyngitis with rash is viral—scarlet fever requires antibiotics, and untreated GAS can lead to serious complications. 1, 3
• Do not rely on clinical impression alone—even experienced physicians cannot differentiate bacterial from viral pharyngitis with certainty; only 35-50% of cases with all clinical features of streptococcal pharyngitis are actually GAS-positive. 5, 1
• Do not skip the backup throat culture after a negative RADT in children/adolescents—this misses 10-20% of GAS infections. 1
• Do not stop antibiotics early—incomplete courses fail to eradicate GAS and do not prevent rheumatic fever. 1, 2

Special Consideration: Amoxicillin-Associated Rash

If you were already started on amoxicillin empirically and the rash appeared 7-20 days later (not hours), consider a benign amoxicillin-viral interaction rather than scarlet fever. 1, 7

• This delayed maculopapular rash occurs in <7% of patients on aminopenicillins, especially with concurrent viral infections (30-100% with Epstein-Barr virus). 1, 7
• This is not a true penicillin allergy—discontinue amoxicillin, provide symptomatic care, and do not label the patient as penicillin-allergic without formal evaluation. 1, 7
• However, your rash appeared within hours of symptom onset, making this scenario unlikely; scarlet fever remains the diagnosis. 1

When to Refer or Escalate

• Severe systemic symptoms, respiratory distress, or signs of sepsis warrant immediate hospitalization. 1
• Persistent fever or worsening symptoms after 48 hours of appropriate antibiotics suggest complications (peritonsillar abscess, retropharyngeal abscess) or an alternative diagnosis. 1
• Development of petechiae, purpura, or rash involving palms/soles requires urgent re-evaluation for RMSF or meningococcemia. 1, 6