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Question and answer links
- W1: A 6-year-old girl has cola-colored urine and puffy eyes four weeks after impetigo on her legs. Which test is most likely · Answer
- W2: A 5-year-old girl has had four days of a raised, non-blanching purple rash over her buttocks and the backs of her legs, · Answer
- Q3: Mateo, 7, has puffy eyes, cola-colored urine, less urine than usual, 1.9 kg of new weight, BP 128/84 (repeat 126/84) aga · Answer
- Q4: Mateo's creatinine is 0.9 mg/dL today; it was 0.4 mg/dL ten months ago, and the lab flags nothing because it printed its · Answer
- Q5: On the ward: BP 126/84, edema, less urine, creatinine 0.9 (2.25 times baseline), potassium 5.3 with a normal ECG, C3 38 · Answer
- Q6: Before discharge, his parents ask what should bring Mateo back early. Which of these developments over the coming weeks · Answer
- Q7: If you were thinking "uncomplicated post-streptococcal GN," and you then learn that at ten weeks — with his BP and creat · Answer
Session S3 · 2026-10-28 · supplementary · 20 min Anchor: A puffy-faced child — post-streptococcal GN, minimal change disease or IgA vasculitis (HSP)? · Integrates: pediatric BP percentiles, AKI staging and hyperkalemia in a child, complement serology, supportive management (salt, fluid, loop diuretic, BP) One-line hook: A puffy-faced 7-year-old who looks like the cousin who got better on prednisone, a blood pressure his parents were told is "normal for an adult," and a creatinine the lab did not flag. Each one has to be read against a second-grader's own numbers before anyone writes a prescription. Self-study after Session 3; commit to each answer before you open it.
Learning Objectives
Answers for this section: W1 · W2 · Q3 · Q4 · Q5 · Q6 · Q7
- Separate a nephritic from a nephrotic child at the bedside, and distinguish post-streptococcal GN (PSGN) from minimal change disease and IgA vasculitis (HSP).
- Classify a child's BP against the age-sex-height table, and stage AKI in a child against his own baseline rather than an adult reference range.
- Order and read the complement pair and the streptococcal antibodies, choosing anti-DNase B after a skin infection.
- Manage PSGN supportively — salt and fluid limits, a loop diuretic, potassium control, an antibiotic course — and name the drugs and procedures it does not need.
- Recognize the developments that need same-day escalation, and the complement timeline that sends a child to nephrology.
Warm-Up (individual commit before the case)
Answers for this section: W1 · W2
W1After a skin infection
A 6-year-old girl has cola-colored urine and puffy eyes four weeks after impetigo on her legs. Which test is most likely to show that she had a recent streptococcal infection?
Antistreptolysin O (ASO) titer, drawn today and repeated in two weeks
Serum C3 level, drawn today and repeated at the six-week visit
Throat culture for group A streptococcus, taken today in the clinic
Anti-DNase B titer, drawn today and then repeated in two weeks
Show answer and explanation · Link to this question
Correct answer: D
Learning point: Both antibodies are made against streptococcal products, but after a skin infection anti-DNase B is the more sensitive marker; ASO is the better marker after a sore throat. A rising titer on the repeat sample is more convincing than a single value — the ASO, for example, peaks 3–5 weeks after the infection. By the time the kidney flares the organism is usually long gone, which is why antibodies, not swabs, carry the history. Why not the others: A — ASO is less sensitive after a skin infection than after pharyngitis; B — a low C3 shows complement being consumed, not which organism started it; C — a throat swab looks for organisms today, not for a skin infection a month ago. Course reading: Evidence for this case
W2The rash on the legs
A 5-year-old girl has had four days of a raised, non-blanching purple rash over her buttocks and the backs of her legs, swollen painful ankles and cramping belly pain. She is afebrile and playful. Her urine shows microscopic hematuria and 1+ protein; platelets are 290 ×10³/µL, and C3 and C4 are normal. What is the most likely diagnosis?
Immune thrombocytopenia, bleeding into the skin and the urine
Meningococcal sepsis, with early purpura and seeding of the joints
IgA vasculitis (Henoch-Schönlein purpura) involving the kidney
Post-streptococcal GN, with a viral rash and a reactive arthritis
Show answer and explanation · Link to this question
Correct answer: C
Learning point: Purpura you can feel, on dependent skin, with joint pain, abdominal pain and kidney involvement is the tetrad of IgA vasculitis — inflamed small vessels, not a shortage of platelets, so the count is normal. It peaks between ages 2 and 15, the kidney is involved in about 40–50% of children, and complement stays normal, which separates it from post-streptococcal GN. Its kidney lesion is the same IgA deposit seen in IgA nephropathy. Why not the others: A — purpura from low platelets needs a low platelet count, and hers is normal; B — an afebrile, playful child four days into a rash is not septic; D — post-streptococcal GN lowers C3 and does not cause purpura. Course reading: Evidence for this case
Part 1 — Presentation
Answers for this section: Q3
A Thursday in October, a pediatric clinic. Mateo is 7 and in second grade. For three mornings his eyes have been puffy; for two days his urine has looked like cola, and he has been going to the bathroom less. Last night his parents took him to an urgent care, where the triage nurse measured his BP at 128/84 and told them it was "normal for an adult." They came in anyway.
Four weeks ago he had impetigo — honey-crusted sores on both shins — treated with mupirocin ointment for five days; the crusts were gone two weeks ago. No sore throat this fall, no rash now, no joint or belly pain, no headache, no vomiting. Last spring his cousin Sofía, then 5, had the same puffy eyes: her urine had no blood, her BP was normal, and she got better on prednisone.
Vitals: T 36.9 °C · HR 96 · RR 20 · BP 128/84 (right arm, child cuff, auscultation after rest; repeat 126/84) · SpO₂ 99% · weight 26.0 kg (24.1 kg at a well visit 2 months ago) · height 125 cm (50th percentile). For a 7-year-old boy at the 50th height percentile, the AAP 2017 table gives a 90th percentile of 109/70, a 95th of 112/73 and a 95th + 12 mmHg line of 124/85.
Exam: Periorbital edema; 1+ pitting edema at both ankles; healed pink scars on both shins. Lungs clear, no gallop, no enlarged liver. No purpura, no joint swelling; abdomen soft and nontender. Alert and comfortable.
| Drug | Dose | Route | Frequency | Indication |
|---|---|---|---|---|
| Mupirocin 2% ointment | thin layer | topical | three times daily for 5 days — finished 3½ weeks ago | Impetigo |
| No other medicines | — | — | — | No NSAIDs, no supplements |
Office urine dipstick: blood 3+, protein 2+, leukocyte esterase and nitrite negative.
Q3Which child is this?
Mateo, 7, has puffy eyes, cola-colored urine, less urine than usual, 1.9 kg of new weight, BP 128/84 (repeat 126/84) against a 95th percentile of 112/73 and a stage 2 line of 124/85, and impetigo four weeks ago. Which reading of his presentation is right?
Nephritic syndrome with stage 2 BP; same-day microscopy, chemistries, complements and strep antibodies
Nephrotic syndrome like his cousin's, with a BP normal for an adult; start prednisone and recheck in a week
IgA vasculitis with a BP in the elevated range only; check a platelet count and arrange a skin biopsy
A urinary tract infection with anxious, painful voiding; start cephalexin and repeat the dipstick in 10 days
Show answer and explanation · Link to this question
Correct answer: A
Learning point: Blood in the urine, less urine, new swelling and a high BP are the nephritic pattern — an inflamed glomerulus leaking red cells and holding salt and water — not the nephrotic pattern (massive protein, very low albumin, bland urine) his cousin had. Under 13, BP is read against age and height: 128 systolic is past his stage 2 line of 124 (and a diastolic of 84 is not normal even in an adult), so with nephritis it is worked up today. Impetigo four weeks ago fits post-streptococcal GN, which follows a skin infection by 3–6 weeks (a throat infection by 1–3). Why not the others: B — the nephrotic label is wrong, and 128/84 is not normal at any age; C — he has no purpura, joint or belly pain, and his BP is past stage 2, not "elevated"; D — leukocyte esterase and nitrite are negative, and cola urine with edema is not cystitis. Course reading: Evidence for this case
Part 2 — Data
Answers for this section: Q4 · Q5
Blood (11:30) beside the only earlier values — pre-anesthesia labs before dental surgery 10 months ago
| Test | Today | 10 months ago | Reference range |
|---|---|---|---|
| Sodium | 136 | 139 | 135–145 mEq/L |
| Potassium | 5.3 | 4.2 | 3.5–5.0 mEq/L |
| Chloride | 104 | 105 | 98–107 mEq/L |
| Bicarbonate | 21 | 24 | 22–29 mEq/L |
| Anion gap, Na − (Cl + HCO₃) | 11 | 10 | 8–12 mEq/L |
| BUN | 18 | 10 | 5–18 mg/dL |
| Creatinine | 0.9 | 0.4 | the lab printed its adult range, 0.6–1.2 mg/dL |
| Albumin | 3.4 | — | 3.5–5.0 g/dL |
| Hemoglobin | 11.2 | 12.6 | 11.5–15.5 g/dL |
| Platelets | 318 | — | 150–450 ×10³/µL |
| C3 | 38 | — | 90–180 mg/dL |
| C4 | 24 | — | 16–47 mg/dL |
| ASO | 280 | — | <200 IU/mL |
| Anti-DNase B | 1,120 | — | <240 U/mL |
| ECG | sinus rhythm 94/min, normal intervals, no peaked T waves | — | — |
Urinalysis with microscopy (fresh clean-catch, read within 30 minutes)
| Parameter | Result | Reference |
|---|---|---|
| Color / specific gravity | brown ("cola") / 1.022 | yellow / 1.005–1.030 |
| Blood / protein | 3+ / 2+ | negative |
| Leukocyte esterase / nitrite | negative / negative | negative |
| RBC | >50/hpf, mostly dysmorphic, acanthocytes present | 0–2/hpf |
| RBC casts | 2–4 per low-power field | none |
| WBC / bacteria | 3–5/hpf / none | 0–5/hpf / none |
| Urine protein-to-creatinine ratio | 1.1 g/g | <0.2 g/g (the child nephrotic line is at least 2 g/g) |
He is admitted to the pediatric ward for blood pressure and urine-output monitoring.
Q4Is 0.9 normal?
Mateo's creatinine is 0.9 mg/dL today; it was 0.4 mg/dL ten months ago, and the lab flags nothing because it printed its adult range of 0.6–1.2. Which statement is correct?
Normal for now: 0.9 sits inside the reference range the laboratory printed with the result
Presumed stage 2 AKI if the rise occurred this week; 0.9 is 2.25 times baseline, and the printed adult range is misleading
KDIGO stage 1: the creatinine is 0.5 mg/dL higher, past the 0.3 mg/dL rise that defines AKI
CKD stage G3a: a bedside Schwartz eGFR of 0.413 × 125 ÷ 0.9 is about 57 mL/min/1.73 m²
Show answer and explanation · Link to this question
Correct answer: B
Learning point: Creatinine comes from muscle, so a 26-kg child runs far lower than an adult: by the bedside Schwartz equation, a normal GFR at 125 cm corresponds to a creatinine of about 0.4–0.5 — which is what he had. Staged against his own baseline, 0.9 ÷ 0.4 = 2.25, KDIGO 2012 stage 2 (2.0–2.9 times baseline); the acute nephritic illness supports presuming the rise happened this week, but a ten-month-old baseline alone cannot establish that interval. Seek intervening results and follow serial creatinine; document the presumption rather than presenting the clock as proven. Until the creatinine stops moving, every printed or calculated eGFR is invalid. Why not the others: A — an adult range hides a doubled creatinine in a child; C — the 0.3 mg/dL criterion needs a 48-hour window, and the higher ratio would determine the stage if the acute interval is supported; D — CKD needs three months, and the Schwartz equation, like any eGFR, needs a steady creatinine. Course reading: Evidence for this case
Q5Today's orders
On the ward: BP 126/84, edema, less urine, creatinine 0.9 (2.25 times baseline), potassium 5.3 with a normal ECG, C3 38 with a normal C4, anti-DNase B 1,120, RBC casts and UPCR 1.1 g/g. Which plan is right for today?
Lisinopril for the blood pressure and the protein, with a repeat potassium level in one week
A kidney biopsy before any treatment, to prove post-streptococcal GN and to look for crescents
Prednisone at nephrotic doses — the same treatment that put his cousin Sofía into remission
Salt and fluid limits, furosemide, avoid potassium supplements, eradicate streptococci, and monitor the BMP
Show answer and explanation · Link to this question
Correct answer: D
Learning point: Post-streptococcal GN usually has a favorable course in children, so treatment supports him while the immune complexes clear: salt and fluid limits and a loop diuretic remove the retained volume that drives both the edema and the BP, and the diuretic also helps the kidney excrete potassium. A potassium of 5.3 with a normal ECG needs no calcium — avoid potassium supplements and potassium-containing IV fluids, individualize dietary restriction, and repeat potassium promptly according to its trend and clinical status. Penicillin for 10 days clears any remaining streptococci; it does not treat the nephritis. Why not the others: A — an ACE inhibitor raises potassium and is held while the creatinine is moving and the potassium is above 5.0 (it can come later if protein persists); B — classic PSGN is not biopsied unless it is atypical or rapidly progressive; C — immunosuppression has no role in routine PSGN. Course reading: Evidence for this case
Part 3 — Course and Decisions
Answers for this section: Q6 · Q7
By hospital day 3 Mateo has lost 1.6 kg. His BP is 108/66 on furosemide alone, his potassium 4.4 and his creatinine 0.7. He goes home on day 4 with a low-salt diet, the rest of his penicillin and a clinic visit in one week.
Q6What brings him back today
Before discharge, his parents ask what should bring Mateo back early. Which of these developments over the coming weeks needs same-day evaluation and a call to pediatric nephrology, rather than his routine visits?
Microscopic red cells at follow-up, after visible discoloration resolved and BP and kidney function improved
An antistreptolysin titer that remains elevated three weeks after admission
A severe headache with vomiting, or a day of far less urine than usual
A trace of protein on the dipstick at his six-week follow-up visit
Show answer and explanation · Link to this question
Correct answer: C
Learning point: The dangerous turns of post-streptococcal GN are hypertensive encephalopathy (headache, vomiting, confusion or a seizure), a falling urine output with a rising creatinine (a nephritis that is not settling, or a rare rapidly progressive course), and fluid in the lungs — each means a same-day visit and a call to pediatric nephrology. The rest is expected recovery: the cola color fades first, microscopic blood can linger for months, protein usually clears within 3–6 months, and streptococcal antibodies can remain elevated after the infection. Why not the others: A — isolated microscopic blood can persist; prolonged or recurrent visible blood needs reassessment; B — an antibody can remain elevated after the infection and does not alone establish recurrent nephritis; D — low-grade protein normally lingers for months. Course reading: Evidence for this case
Ten weeks after his eyes first puffed up, Mateo is back for a scheduled visit and looks well. BP 102/64. Creatinine 0.45 mg/dL. Urine: trace protein, 8–10 RBC/hpf, no casts; UPCR 0.3 g/g. A C3 and C4 are drawn that morning.
Q7Evidence update
Concordance-style practice: there is no expert panel behind this key, the teaching explanation. "Unchanged" means the finding does not move the hypothesis — not "I'm not sure."
If you were thinking "uncomplicated post-streptococcal GN," and you then learn that at ten weeks — with his BP and creatinine back to normal — his C3 is still low at 52 mg/dL (reference 90–180; C4 26, normal), this hypothesis becomes:
Much less likely
Less likely
Unchanged
More likely
Much more likely
Show answer and explanation · Link to this question
Correct answer: B
Learning point: Persistent low C3 at ten weeks is atypical enough to reconsider an uncomplicated postinfectious course and arrange pediatric nephrology review now. C3 often normalizes within roughly 6–8 weeks, but no single date proves C3 glomerulopathy. Repeat complement, proteinuria and kidney-function assessment and let the full trajectory guide biopsy. Why not the others: A is also defensible if the learner explains the prolonged complement abnormality; C ignores the unexpected persistence; D and E strengthen the wrong interpretation. Twelve weeks is not a reason to defer evaluation. Course reading: Evidence for this case
Resolution and Teaching Points
Answers for this section: W1 · W2 · Q3 · Q4 · Q5 · Q6 · Q7
At 12 weeks Mateo's C3 was 49 mg/dL. Pediatric nephrology biopsied him (BP and platelets normal) and diagnosed C3 glomerulopathy — a disease of an overactive alternative complement pathway that can begin, as his did, looking exactly like post-streptococcal GN. Its treatment is nephrology's; he is seen in their clinic with a urinalysis, UPCR, creatinine and BP at every visit. His parents kept the urgent-care printout. They now ask for "his percentile, not the adult number."
Clinical Pearl — Nephritic or nephrotic first, then the clock Blood, high BP, less urine and edema are nephritic; massive protein, very low albumin and a bland urine are nephrotic. Then the clock sorts the nephritic child: 1–3 days after a sore throat is IgA; 1–3 weeks after a throat infection or 3–6 weeks after a skin infection is post-streptococcal GN.
Clinical Pearl — Read the child's own numbers Until 13, BP is staged against the age-sex-height table. Creatinine is read against the child's own baseline, or checked against 0.413 × height ÷ creatinine — an adult reference range can hide a doubled creatinine.
Clinical Pearl — Complement before names Low C3 with a normal C4: post-infectious GN or C3 glomerulopathy. Both low: lupus, cryoglobulinemia, endocarditis, MPGN. Both normal: IgA nephropathy and IgA vasculitis, ANCA vasculitis, anti-GBM disease. After a skin infection, order anti-DNase B.
Clinical Pearl — PSGN is supportive care Salt and fluid limits, a loop diuretic, potassium control and an antibiotic to clear the streptococci. No steroids, no biopsy unless it is atypical, and no ACE inhibitor while the potassium is above 5.0 and the creatinine is moving.
Clinical Pearl — The C3 clock C3 usually recovers over several weeks. Persistent depression at 8–12 weeks warrants nephrology reassessment for alternatives; the timing supports a work-up rather than proving one diagnosis.
Pitfall — "Normal for an adult" The triage nurse was wrong twice: a diastolic of 84 is stage 1 even by adult lines, and at his height 128 systolic is stage 2. An adult creatinine range hides a sick child the same way. Ask for his percentile and his baseline.
Pitfall — Treating the cousin Prednisone for the next puffy child in the family, written before anyone looked at his urine, his BP or his complement.
Concept Links
Answers for this section: W1 · W2 · Q3 · Q4 · Q5 · Q6 · Q7
Assigned reading: the explanations in this case and the question-linked course evidence guide. The objective map distinguishes tested decisions from concepts taught in explanations.
Concepts: aki-01, aki-02, aki-24, glom-07, glom-12, glom-13, glom-17, glom-23, htn-cardiorenal-17, lytes-13.