Which of the following is the most likely diagnosis?

Author

Dr. Shoaib Ahmad Baig
January 18, 2025

🔍 Question

A 10-year-old boy is brought to the office due to dark brown urine that he first noticed yesterday after swimming practice.

Blood pressure is 130/80 mm Hg. Physical examination is normal with the exception of bilateral periorbital edema. Laboratory results are as follows:

Serum chemistry

Parameters

Value

Sodium

140 mEq/L

Potassium

4 mEq/L

Blood urea nitrogen

14 mg/dL

Creatinine

1.4 mg/dL

Creatine kinase:

86 U/L

Urinalysis

Protein

1+

Leukocyte esterase

negative

Nitrites

negative

White blood cells:

1-2/hpf

Red blood cells:

many/hpf

Casts

RBC casts

Which of the following is the most likely diagnosis?

A. Minimal change disease

B. Nephrolithiasis

C. Postinfectious glomerulonephritis

D. Pyelonephritis

E. Rhabdomyolysis

Correct Answer 🎯:C. Postinfectious glomerulonephritis

⚙️

🧠 Trigger: Recent Streptococcal infection (skin/pharynx)

⬇

Formation of immune complexes (antigen-antibody complexes)

⬇

Deposition of immune complexes in glomerular basement membrane

⬇

Activation of complement system

⬇

Inflammation and glomerular injury

⬇

Symptoms Explained 🔧

1. Glomerular capillary wall damage

⬇

RBCs leak into urine → Hematuria (brown urine)

⬇

RBC casts form in tubules → RBC casts on urinalysis

2. Increased glomerular permeability

⬇

Protein leakage → Mild proteinuria

3. Glomerular inflammation

⬇

Reduced GFR → Fluid retention

⬇

Bilateral periorbital edema

4. Systemic effects

⬇

Sodium/water retention → Hypertension

How To Approach this Question? 🏗️

Focus on immune-mediated glomerular injury due to recent infection (PIGN hallmark).

• Dark urine → indicates hematuria with RBC casts.

• Edema & mild proteinuria → glomerular damage.

• Eliminate options based on absence of key findings like proteinuria, hematuria with casts, or systemic features.

⚖️ Explanation of Other Differentials

Differential ⚖️

Mechanism ⚙️

Why Incorrect? 🔴

Minimal Change Disease

Loss of podocyte foot processes.

Would present with severe proteinuria (>3.5 g/day) (nephrotic syndrome).

• No hematuria or RBC casts.

Nephrolithiasis

Stone obstructing the urinary tract.

Presents with colicky flank pain and hematuria but no RBC casts or proteinuria.

• Edema and hypertension are not features.

Pyelonephritis

Ascending bacterial infection.

• Presents with fever, flank pain, pyuria, and WBC casts.

• No RBC casts, edema, or recent infection link.

Rhabdomyolysis

Muscle breakdown releasing myoglobin.

• Urine would be dark due to myoglobinuria, not RBCs.

• No RBC casts or signs of glomerular inflammation.

📝 Flashcards

Stay Hungry, Stay Curious!

Your Brother In This Struggle

Dr. Shoaib Ahmad

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