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A 9-year-old boy is brought by his mother with cola-coloured urine, facial swelling (worse in the morning) and headache for 3 days. Two weeks ago he was treated for a sore throat with oral penicillin V for 5 days. On examination: BP 150/96 mmHg (above the 99th centile), periorbital and pedal oedema, JVP raised 4 cm, basal crackles. Urine dipstick: blood 3+, protein 1+. Phase-contrast microscopy shows dysmorphic red cells and red-cell casts. Serum creatinine 1.4 mg/dL (raised for age); potassium 5.6 mmol/L; C3 38 mg/dL (low), C4 normal; ASO titre 800 Todd units (raised); anti-DNase B raised. Anti-GBM, ANA, anti-dsDNA and ANCA negative; hepatitis B/C and HIV negative.
Questions
a) What is the most likely diagnosis, and name two key investigations that support it? (2 marks)
Post-streptococcal glomerulonephritis (PSGN), the archetype of acute immune-complex proliferative glomerulonephritis. Two supporting features: (i) the low serum C3 with normal C4 (alternative/lectin pathway activation) and (ii) the raised ASO and anti-DNase B titres confirming recent group A streptococcal pharyngitis, with the characteristic 1- to 3-week latent period.
b) Outline the mechanism of the renal injury and explain the hypertension. (3 marks)
Nephritogenic streptococcal antigens (SpeB and NAPlr) deposit in glomeruli (in situ and as circulating immune complexes), forming characteristic subepithelial humps. Complement is activated via the alternative and lectin pathways, consuming C3; neutrophils and macrophages infiltrate the tuft and mesangial/endothelial cells proliferate (diffuse endocapillary proliferative GN on light microscopy). The inflamed capillary wall ruptures, allowing red cells and RBC casts to leak (the active sediment). The reduced GFR with intact tubular sodium reabsorption causes sodium and water retention — the 'overfill' mechanism of oedema — and intrarenal renin-angiotensin system activation; both produce volume-mediated hypertension and an acute kidney injury with oliguria and hyperkalaemia.
c) Outline the immediate and definitive management. (3 marks)
Immediate (resuscitative): control the hypertensive emergency with gradual BP reduction (under 25% in the first hour then to 160/100 over 2 to 6 hours) using an IV agent such as labetalol 20 to 80 mg IV bolus or nicardipine infusion (avoid sublingual nifedipine); treat hyperkalaemia with calcium gluconate, insulin-dextrose and salbutamol; treat pulmonary oedema with oxygen, sit upright and IV furosemide 40 to 80 mg. Definitive (PSGN is supportive and self-limiting): salt restriction under 2 g/day, fluid restriction to urine output plus insensible losses, loop diuretic for oedema, and a 10-day course of oral penicillin V (or erythromycin if allergic) to clear the streptococcus — this does NOT change the GN course but prevents rheumatic fever and spread. Dialysis is reserved for refractory hyperkalaemia, acidosis, pulmonary oedema or uraemia (AEIOU).
d) What is the expected prognosis, and which feature would prompt renal biopsy? (2 marks)
Prognosis is excellent in children: over 95% have complete recovery; C3 normalises within 6 to 8 weeks; microscopic haematuria may persist for up to a year. Biopsy is prompted by atypical features: persistent low C3 beyond 8 weeks (suggesting MPGN or C3 glomerulopathy), nephrotic-range proteinuria, a rapidly progressive course with crescents on biopsy, or no clear post-infectious trigger — each mandates urgent renal biopsy to redefine management.