What is the most likely diagnosis in a patient with a 3‑day history of sore throat and undocumented fever, pink urine unrelated to menstruation, a prior high antistreptolysin‑O (ASO) titer, and recent treatment with Sumapen (ampicillin‑sulbactam)?

Most Likely Diagnosis: Post-Streptococcal Glomerulonephritis (PSGN)

This patient most likely has post-streptococcal glomerulonephritis (PSGN), given the combination of recent streptococcal pharyngitis (evidenced by prior high ASO titer), current sore throat with fever, and pink urine unrelated to menstruation.

Clinical Reasoning

Key Diagnostic Features

The constellation of findings strongly suggests PSGN:

• Pink/tea-colored urine (hematuria) is a hallmark of acute glomerulonephritis and typically appears 1-3 weeks after streptococcal pharyngitis 1
• Prior elevated ASO titer confirms recent group A streptococcal infection, which is the most common trigger for PSGN 2
• Current sore throat with fever may represent either a new streptococcal infection or the tail end of the initial infection that triggered the glomerulonephritis 1
• History of treatment with Sumapen (ampicillin-sulbactam) indicates recent bacterial pharyngitis requiring antibiotic therapy 3

Understanding ASO Titers in This Context

The elevated ASO titer is particularly significant here:

• ASO titers peak 2-4 months after acute streptococcal infection and remain elevated for months 2
• In acute rheumatic fever, ASO titers show remarkable response during the acute phase, but ASO titers are not recommended for diagnosing acute pharyngitis as they reflect past, not current events 1
• The prior high ASO titer confirms preceding streptococcal exposure, which is the critical link to PSGN 4

Differential Considerations

While the current sore throat could represent:

1. Recurrent streptococcal pharyngitis - possible, but the pink urine is the key distinguishing feature that points to renal involvement 1
2. Viral pharyngitis - common, but doesn't explain the hematuria 1
3. Treatment failure - ampicillin-sulbactam is effective against group A streptococcus, making this less likely 3

The pink urine is the critical finding that elevates this beyond simple pharyngitis to a post-infectious complication.

Immediate Diagnostic Workup Required

To confirm PSGN and assess severity:

• Urinalysis with microscopy - looking for red blood cells, red cell casts (pathognomonic for glomerulonephritis), and proteinuria 4
• Serum creatinine and BUN - to assess renal function
• Complement levels (C3, C4) - typically low C3 in PSGN, normalizes in 6-8 weeks
• Blood pressure measurement - hypertension is common in PSGN
• Throat culture or rapid antigen test - to determine if current streptococcal infection is present 1

Regarding the Current Pharyngitis

For the sore throat component:

• Clinical scoring (Centor criteria) or rapid antigen testing should guide antibiotic decisions for the current pharyngitis 1
• If testing confirms group A streptococcus, complete a full 10-day course of penicillin V or amoxicillin 1
• The fact that Sumapen was already given suggests bacterial pharyngitis was suspected or confirmed previously 3

Critical Management Considerations

Pitfalls to Avoid

• Do not dismiss the pink urine as insignificant - this is the red flag for renal involvement and requires immediate evaluation
• Do not rely on ASO titers for diagnosing acute pharyngitis - they reflect past infection 1
• Do not assume the pink urine is related to the antibiotic - while ampicillin-sulbactam can cause various side effects, hematuria in this context with prior streptococcal infection points to PSGN 3

Morbidity and Mortality Implications

PSGN, while typically self-limited in children, requires monitoring because:

• Acute complications include hypertensive encephalopathy, acute renal failure, and fluid overload
• Most patients recover completely, but a small percentage may develop chronic kidney disease
• Early recognition and supportive management significantly impact outcomes

Immediate referral to nephrology or urgent care evaluation is warranted given the combination of hematuria and recent streptococcal infection, as this represents a potentially serious post-infectious complication requiring close monitoring and supportive care.