Causative Organism: Streptococcus pyogenes (Group A β-Hemolytic Streptococcus)
The most likely causative organism is Streptococcus pyogenes (Group A β-hemolytic streptococcus), which causes scarlet fever—the classic presentation of pharyngitis, fever, sandpaper-like rash, and strawberry tongue. 1
Clinical Reasoning
This patient presents with the pathognomonic features of scarlet fever:
- Pharyngitis with fever: Group A streptococcal pharyngitis typically presents with sudden-onset sore throat, pain on swallowing, and fever (often 101-104°F) 2, 3
- Sandpaper-like rash: The scarlatiniform rash is caused by streptococcal pyrogenic exotoxins produced by certain strains of S. pyogenes 1. The rash typically begins on the trunk and spreads to extremities, with a characteristic sandpaper texture 4
- Strawberry tongue: This distinctive finding—initially white-coated then bright red with prominent papillae—is virtually diagnostic when combined with the other features 3
Why Not the Other Options?
Staphylococcus aureus (Option B) causes toxic shock syndrome, which can present with fever and diffuse erythematous rash, but typically lacks the prominent pharyngitis and strawberry tongue seen here 2. Additionally, staphylococcal toxic shock presents with hypotension and multi-organ involvement, which are not described in this case.
Parvovirus (Option C) causes erythema infectiosum (fifth disease) with a "slapped cheek" appearance and lacy reticular rash, but does not cause the sandpaper-like texture, strawberry tongue, or severe pharyngitis characteristic of this presentation 4.
Diagnostic Confirmation Required
Despite the classic presentation, microbiological confirmation is essential before initiating antibiotics. 2, 1
- Obtain a rapid antigen detection test (RADT) or throat culture from the posterior pharynx and tonsillar surfaces 4
- In children and adolescents, negative RADT should be confirmed with throat culture (gold standard) 1
- Clinical scoring systems predict positive results only ≤80% of the time, so laboratory confirmation is mandatory 3
Critical Pitfall to Avoid
Do not rely solely on clinical diagnosis, even with classic features. The Infectious Diseases Society of America emphasizes that even experienced physicians cannot diagnose streptococcal pharyngitis with certainty based on clinical grounds alone—bacteriologic confirmation is required 2. Waiting for "classic" presentation can delay treatment and increase complications including acute rheumatic fever, glomerulonephritis, and endocarditis 3.
Immediate Management
Once diagnosis is confirmed:
- Initiate oral Penicillin V 250-500 mg every 6-8 hours for 10 days 4
- For penicillin allergy (non-anaphylactic): first-generation cephalosporin 4
- For true penicillin allergy (anaphylactic): macrolides (though be aware of potential resistance) 4
- Patient becomes non-contagious after 24 hours of antibiotic therapy 4
- Antibiotics can be started up to 9 days after symptom onset and still prevent rheumatic fever 4