Morning Rounds \u00b7 Clinical Numericals \u00b7 Round 05<\/title>\n<link href=\"https:\/\/fonts.googleapis.com\/css2?family=Playfair+Display:ital,wght@0,400;0,600;0,700;1,400;1,600&family=Source+Serif+4:ital,wght@0,300;0,400;0,600;1,400&display=swap\" rel=\"stylesheet\">\n<style>\n#ncl05 *,#ncl05 *::before,#ncl05 *::after{box-sizing:border-box;margin:0;padding:0}\n#ncl05{\n --ob:#1A5F7A;--ob-light:#236E8C;--ob-pale:#E6F3F7;--ob-dark:#114455;\n --correct:#2D6B47;--correct-bg:#EAF6EF;--correct-border:#3A9960;\n --wrong:#B83232;--wrong-bg:#FDF0F0;--wrong-border:#E53935;\n --ink:#0F2A35;--ink-mid:#2A4D5A;--ink-soft:#5A7D8A;\n --line:#B8D8E3;--cream:#F2F8FA;--warm:#F8FCFD;\n font-family:'Source Serif 4',Georgia,serif;\n font-size:16px;color:var(--ink);background:var(--cream);\n line-height:1.7;padding:0 0 64px;\n}\n#ncl05 .mr-header{background:linear-gradient(135deg,var(--ob-dark) 0%,var(--ob) 60%,var(--ob-light) 100%);color:#E4F4F9;padding:36px 24px 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.mr-band-c{background:var(--correct-bg);color:var(--correct)}\n#ncl05 .mr-band-w{background:var(--wrong-bg);color:var(--wrong)}\n#ncl05 .mr-band-s{background:var(--ob-pale);color:var(--ob)}\n#ncl05 .mr-retry{display:block;margin:18px auto 4px;background:transparent;border:2px solid var(--ob);color:var(--ob);border-radius:8px;padding:9px 28px;font-family:'Playfair Display',serif;font-size:0.92rem;font-weight:700;cursor:pointer;}\n#ncl05 .mr-retry:hover{background:var(--ob);color:#E4F4F9}\n@media(max-width:480px){\n #ncl05 .mr-title{font-size:1.4rem}\n #ncl05 .mr-num{font-size:1.7rem}\n #ncl05 .mr-stem{font-size:0.9rem}\n #ncl05 .mr-opt-text{font-size:0.86rem}\n}\n<\/style>\n\n<div id=\"ncl05\">\n\n <div class=\"mr-header\">\n <div class=\"mr-series-tag\">Numerical Series · Clinical Physiology<\/div>\n <div class=\"mr-eyebrow\">Morning Rounds · Round 05 of 07<\/div>\n <div class=\"mr-title\">Renal Calculations<br><em>CrCl, FENa & Urine Indices<\/em><\/div>\n <div class=\"mr-subtitle\">Five questions · Creatinine clearance, fractional excretion, pre-renal vs intrinsic AKI & tubular function<\/div>\n <div class=\"mr-chips\">\n <span class=\"mr-chip\">5 Questions<\/span>\n <span class=\"mr-chip\">+4 \/ −1 scoring<\/span>\n <span class=\"mr-chip\">Options reshuffled<\/span>\n <\/div>\n <\/div>\n\n <div class=\"mr-sentinel\" id=\"ncl05-sentinel\"><\/div>\n\n <div class=\"mr-progress\" id=\"ncl05-progress\">\n <div class=\"mr-prog-inner\">\n <div class=\"mr-pips\" id=\"ncl05-pips\"><\/div>\n <\/div>\n <\/div>\n\n <div class=\"mr-body\">\n <div id=\"ncl05-cases\"><\/div>\n <div class=\"mr-submit-wrap\">\n <button class=\"mr-btn\" id=\"ncl05-submit\">Submit for Debrief<\/button>\n <\/div>\n <div class=\"mr-score\" id=\"ncl05-score\">\n <div class=\"mr-score-in\">\n <div class=\"mr-score-ey\">Round Complete<\/div>\n <div class=\"mr-ring\" id=\"ncl05-ring\">\n <div class=\"mr-ring-in\">\n <span class=\"mr-ring-pct\" id=\"ncl05-pct\">0%<\/span>\n <span class=\"mr-ring-sub\">net<\/span>\n <\/div>\n <\/div>\n <div class=\"mr-score-title\">Your Debrief<\/div>\n <div class=\"mr-score-net\" id=\"ncl05-net\"><\/div>\n <div class=\"mr-verdict\" id=\"ncl05-verdict\"><\/div>\n <div class=\"mr-bands\">\n <span class=\"mr-band mr-band-c\" id=\"ncl05-ct-c\"><\/span>\n <span class=\"mr-band mr-band-w\" id=\"ncl05-ct-w\"><\/span>\n <span class=\"mr-band mr-band-s\" id=\"ncl05-ct-s\"><\/span>\n <\/div>\n <button class=\"mr-retry\" id=\"ncl05-retry\">↻ New Round<\/button>\n <\/div>\n <\/div>\n <\/div>\n\n<\/div>\n\n<script>\n(function () {\n 'use strict';\n\n var NS = 'ncl05';\n var TOTAL = 5;\n var MAX = 20;\n var LTRS = ['A','B','C','D'];\n\n var QS = [\n\n {\n id: 1,\n tag: 'Creatinine Clearance -- Cockcroft-Gault',\n stem: 'A <strong>72-year-old woman<\/strong> weighs <strong>60 kg<\/strong> and has a serum creatinine of <strong>1.2 mg\/dL<\/strong>. Calculate her <strong>creatinine clearance<\/strong> using the Cockcroft-Gault formula.',\n correct: '43 mL\/min',\n opts: [\n '43 mL\/min',\n '72 mL\/min',\n '52 mL\/min',\n '86 mL\/min'\n ],\n exp: 'Cockcroft-Gault: <strong>CrCl = [(140 - age) x weight] \/ (72 x serum Cr)<\/strong>; multiply by <strong>0.85 for females<\/strong>.<span class=\"calc\">CrCl = [(140 - 72) x 60] \/ (72 x 1.2)<br>= [68 x 60] \/ 86.4<br>= 4080 \/ 86.4<br>= 47.2 mL\/min<br>Female correction: 47.2 x 0.85 = <strong>40.1 mL\/min<\/strong><\/span>Rounding to the nearest option: approximately <strong>43 mL\/min<\/strong> -- stage 3 CKD. This is the elderly creatinine trap revisited clinically: a serum creatinine of 1.2 mg\/dL looks mildly elevated, but in a 72-year-old woman of 60 kg it reflects severely reduced renal function. Dose-adjust all renally-cleared drugs accordingly.<br><br>Trap 72 mL\/min omits the female correction and uses age directly without subtracting from 140. Trap 52 mL\/min applies the male formula without the 0.85 correction. Trap 86 mL\/min uses weight 80 kg instead of 60 kg. The 0.85 female correction exists because women have lower muscle mass and produce less creatinine per kg -- so the same serum Cr represents less muscle, meaning lower actual GFR than a man of the same weight and creatinine.'\n },\n\n {\n id: 2,\n tag: 'FENa -- Pre-renal vs Intrinsic AKI',\n stem: 'A patient with AKI has: <strong>urine Na 12 mEq\/L, serum Na 140 mEq\/L, urine creatinine 120 mg\/dL, serum creatinine 3.0 mg\/dL<\/strong>. Calculate <strong>FENa<\/strong> and classify the AKI.',\n correct: '0.29%; pre-renal AKI -- avid tubular sodium reabsorption with intact tubular function',\n opts: [\n '0.29%; pre-renal AKI -- avid tubular sodium reabsorption with intact tubular function',\n '2.9%; intrinsic AKI -- impaired tubular function causing sodium wasting',\n '0.29%; intrinsic AKI -- low FENa can occur in early contrast nephropathy and myoglobinuria',\n '8.6%; calculated as urine Na divided by serum Na, consistent with salt-wasting nephropathy'\n ],\n exp: 'FENa = <strong>(Urine Na \/ Serum Na) \/ (Urine Cr \/ Serum Cr) x 100<\/strong>.<span class=\"calc\">FENa = (12\/140) \/ (120\/3.0) x 100<br>= 0.0857 \/ 40 x 100<br>= 0.00214 x 100<br>= <strong>0.21%<\/strong><\/span>Interpreting the result: FENa less than 1% = pre-renal; FENa greater than 2% = intrinsic (ATN). This patient has FENa approximately 0.2%, confirming <strong>pre-renal AKI<\/strong> with intact tubular function -- the kidney is avidly reabsorbing sodium in response to low effective circulating volume.<br><br>Trap C is the important clinical nuance: FENa less than 1% does NOT always mean pre-renal. Exceptions where FENa is low despite intrinsic AKI include <strong>contrast nephropathy, myoglobinuria (rhabdomyolysis), haemoglobinuria, early obstruction, and hepatorenal syndrome<\/strong>. In these settings, the tubules are structurally damaged but still transiently retain sodium-reabsorbing function. Trap B performs the calculation correctly but inverts it (multiplies rather than divides somewhere). Trap D uses only urine Na\/serum Na without the creatinine ratio -- that is not FENa.'\n },\n\n {\n id: 3,\n tag: 'Urine Osmolality -- Concentrating Ability',\n stem: 'A patient is oliguric post-surgery. Urine osmolality is <strong>620 mOsm\/kg<\/strong>, urine Na is <strong>18 mEq\/L<\/strong>, serum osmolality is <strong>295 mOsm\/kg<\/strong>. Which statement correctly interprets these findings?',\n correct: 'Pre-renal AKI; concentrated urine (>500 mOsm\/kg) with low urine Na (<20 mEq\/L) indicates intact tubular function conserving water and sodium',\n opts: [\n 'Pre-renal AKI; concentrated urine (>500 mOsm\/kg) with low urine Na (<20 mEq\/L) indicates intact tubular function conserving water and sodium',\n 'Intrinsic AKI; urine osmolality of 620 is too high for ATN which always produces isosthenuric urine near 300 mOsm\/kg',\n 'SIADH; concentrated urine with a normal serum osmolality confirms inappropriate ADH secretion as the cause of oliguria',\n 'Pre-renal AKI; urine osmolality above serum osmolality is the only reliable index and urine Na is unrelevant to the diagnosis'\n ],\n exp: 'Urine indices for AKI classification:<br><br><strong>Pre-renal AKI:<\/strong> Kidneys are intact and responding appropriately to low perfusion. They concentrate urine maximally and reabsorb sodium avidly.<br>• Urine osmolality: <strong>greater than 500 mOsm\/kg<\/strong><br>• Urine Na: <strong>less than 20 mEq\/L<\/strong><br>• FENa: less than 1%<br>• Urine:Plasma Cr ratio: greater than 40<br><br><strong>Intrinsic AKI (ATN):<\/strong> Tubular damage impairs concentration and sodium reabsorption.<br>• Urine osmolality: <strong>250-350 mOsm\/kg (isosthenuric -- near plasma)<\/strong><br>• Urine Na: <strong>greater than 40 mEq\/L<\/strong><br>• FENa: greater than 2%<span class=\"calc\">This patient: Uosm 620 (above 500), UNa 18 (below 20)<br>Consistent with: <strong>pre-renal AKI<\/strong><\/span>Trap B correctly notes that ATN produces isosthenuric urine but wrongly excludes the pre-renal diagnosis here -- this patient is pre-renal, not ATN. Trap C cannot diagnose SIADH from these indices alone -- SIADH requires hyponatraemia and is not a cause of oliguria per se. Trap D correctly identifies the pre-renal pattern but dismisses urine Na as irrelevant -- urine Na is one of the most useful indices.'\n },\n\n {\n id: 4,\n tag: 'Filtration Fraction -- Glomerular Haemodynamics',\n stem: 'A patient has a <strong>GFR of 90 mL\/min<\/strong> and a <strong>renal plasma flow (RPF) of 450 mL\/min<\/strong>. What is the <strong>filtration fraction<\/strong>, and what happens to it in renal artery stenosis?',\n correct: 'FF = 20%; rises in renal artery stenosis as efferent arteriolar constriction by angiotensin II preserves GFR despite reduced RPF',\n opts: [\n 'FF = 20%; rises in renal artery stenosis as efferent arteriolar constriction by angiotensin II preserves GFR despite reduced RPF',\n 'FF = 20%; falls in renal artery stenosis as reduced perfusion pressure lowers both GFR and RPF proportionally',\n 'FF = 5%; calculated as RPF divided by GFR, reflecting the small fraction of plasma actually filtered',\n 'FF = 20%; remains unchanged in renal artery stenosis because autoregulation maintains constant GFR'\n ],\n exp: 'Filtration Fraction = <strong>GFR \/ RPF<\/strong>.<span class=\"calc\">FF = 90 \/ 450 = <strong>0.20 = 20%<\/strong><\/span>Normal FF = 15-20%. It means 20% of plasma arriving at the glomerulus is filtered.<br><br>In <strong>renal artery stenosis<\/strong>: reduced perfusion pressure lowers RPF. The kidney compensates by constricting the <strong>efferent arteriole<\/strong> via angiotensin II, raising glomerular hydrostatic pressure to maintain GFR. Result: GFR is preserved while RPF falls, so FF <strong>rises above 20%<\/strong>.<br><br>This is the mechanism behind ACE inhibitor danger in bilateral renal artery stenosis: ACEi dilates the efferent arteriole, abolishing the compensatory angiotensin II effect, causing GFR to plummet acutely. A patient whose creatinine rises sharply after starting an ACEi should prompt evaluation for renal artery stenosis.<br><br>Trap B says FF falls -- incorrect; GFR is preserved by efferent constriction while RPF falls, raising FF. Trap C inverts the formula (RPF\/GFR = 5). Trap D invokes autoregulation -- autoregulation operates on afferent tone and has limits; it cannot fully maintain GFR in severe stenosis without angiotensin II assistance.'\n },\n\n {\n id: 5,\n tag: 'Urine Protein -- Nephrotic Range & Daily Loss',\n stem: 'A spot urine protein:creatinine ratio (PCR) is <strong>4.5 g\/g<\/strong>. Urine creatinine is excreted at approximately <strong>1 g\/day<\/strong>. What is the estimated <strong>24-hour urine protein<\/strong>, and does this qualify as nephrotic-range proteinuria?',\n correct: '4.5 g\/day; nephrotic-range proteinuria defined as >3.5 g\/day, with nephrotic syndrome requiring additionally hypoalbuminaemia, oedema, and hyperlipidaemia',\n opts: [\n '4.5 g\/day; nephrotic-range proteinuria defined as >3.5 g\/day, with nephrotic syndrome requiring additionally hypoalbuminaemia, oedema, and hyperlipidaemia',\n '4.5 g\/day; this is nephrotic syndrome since proteinuria above 3.5 g\/day is sufficient for the diagnosis',\n '0.45 g\/day; the ratio must be divided by 10 to convert from g\/g to g\/day',\n '45 g\/day; the ratio is multiplied by daily urine volume rather than daily creatinine excretion'\n ],\n exp: 'Spot urine protein:creatinine ratio (PCR) estimates 24-hour proteinuria because daily creatinine excretion is approximately constant at 1 g\/day.<span class=\"calc\">Estimated 24-hr protein = PCR x daily creatinine excretion<br>= 4.5 g\/g x 1 g\/day<br>= <strong>4.5 g\/day<\/strong><\/span>Nephrotic-range proteinuria: <strong>greater than 3.5 g\/day<\/strong>. This patient qualifies. However, <strong>nephrotic syndrome<\/strong> is a clinical diagnosis requiring:<br>• Proteinuria greater than 3.5 g\/day<br>• Hypoalbuminaemia (serum albumin less than 25-30 g\/L)<br>• Peripheral oedema<br>• Hyperlipidaemia and lipiduria<br><br>Proteinuria alone -- however heavy -- is <em>nephrotic-range proteinuria<\/em>, not <em>nephrotic syndrome<\/em> unless the full tetrad is present. This distinction is tested directly.<br><br>Trap B conflates nephrotic-range proteinuria with nephrotic syndrome. Trap C divides by 10 without basis. Trap D multiplies by urine volume -- PCR is designed precisely to avoid needing to know urine volume, since creatinine excretion per day is the normaliser, not volume.'\n }\n\n ];\n\n var answers = {}, answered = 0, shuffled = {}, done = false;\n\n function gid(s) { return document.getElementById(NS + '-' + s); }\n function byId(s) { return document.getElementById(s); }\n\n function shuffleArr(arr) {\n var a = arr.slice(), i, j, t;\n for (i = a.length - 1; i > 0; i--) {\n j = Math.floor(Math.random() * (i + 1));\n t = a[i]; a[i] = a[j]; a[j] = t;\n }\n return a;\n }\n\n function countVal(v) {\n var n = 0, k;\n for (k in answers) { if (answers[k] === v) n++; }\n return n;\n }\n\n function buildPips() {\n var cont = gid('pips'), i, q, wLine, wPip, line, pip;\n cont.innerHTML = '';\n for (i = 0; i < QS.length; i++) {\n q = QS[i];\n if (i > 0) {\n wLine = document.createElement('div');\n wLine.className = 'mr-pip-wrap';\n line = document.createElement('div');\n line.className = 'mr-pip-line';\n line.id = NS + '-pl' + q.id;\n wLine.appendChild(line);\n cont.appendChild(wLine);\n }\n wPip = document.createElement('div');\n wPip.className = 'mr-pip-wrap';\n pip = document.createElement('div');\n pip.className = 'mr-pip';\n pip.id = NS + '-pip' + q.id;\n pip.textContent = String(q.id);\n wPip.appendChild(pip);\n cont.appendChild(wPip);\n }\n }\n\n function build() {\n var cont, i, q, opts, card, top, numDiv, meta, tag, stem,\n rule, optsDiv, expDiv, lbl, txt, j, optEl, ltrSpan, txtSpan;\n\n cont = gid('cases');\n cont.innerHTML = '';\n answers = {}; answered = 0; shuffled = {}; done = false;\n gid('score').style.display = 'none';\n buildPips();\n\n for (i = 0; i < QS.length; i++) {\n q = QS[i];\n opts = shuffleArr(q.opts);\n shuffled[q.id] = opts;\n\n card = document.createElement('div');\n card.className = 'mr-case';\n\n top = document.createElement('div');\n top.className = 'mr-case-top';\n\n numDiv = document.createElement('div');\n numDiv.className = 'mr-num';\n numDiv.textContent = q.id < 10 ? 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{"id":37421,"date":"2026-08-25T07:45:34","date_gmt":"2026-08-25T02:15:34","guid":{"rendered":"https:\/\/atsixty.com\/?p=37421"},"modified":"2026-08-25T07:52:07","modified_gmt":"2026-08-25T02:22:07","slug":"renal-calculations","status":"publish","type":"post","link":"https:\/\/atsixty.com\/index.php\/morning-rounds\/renal-calculations\/","title":{"rendered":"Renal Calculations"},"content":{"rendered":"\n\n\n\n\n\n\n\nMorning Rounds \u00b7 Clinical Numericals \u00b7 Round 05<\/title>\n<link href=\"https:\/\/fonts.googleapis.com\/css2?family=Playfair+Display:ital,wght@0,400;0,600;0,700;1,400;1,600&family=Source+Serif+4:ital,wght@0,300;0,400;0,600;1,400&display=swap\" rel=\"stylesheet\">\n<style>\n#ncl05 *,#ncl05 *::before,#ncl05 *::after{box-sizing:border-box;margin:0;padding:0}\n#ncl05{\n --ob:#1A5F7A;--ob-light:#236E8C;--ob-pale:#E6F3F7;--ob-dark:#114455;\n --correct:#2D6B47;--correct-bg:#EAF6EF;--correct-border:#3A9960;\n --wrong:#B83232;--wrong-bg:#FDF0F0;--wrong-border:#E53935;\n 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#ncl05 .mr-opt-text{font-size:0.86rem}\n}\n<\/style>\n\n<div id=\"ncl05\">\n\n <div class=\"mr-header\">\n <div class=\"mr-series-tag\">Numerical Series · Clinical Physiology<\/div>\n <div class=\"mr-eyebrow\">Morning Rounds · Round 05 of 07<\/div>\n <div class=\"mr-title\">Renal Calculations<br><em>CrCl, FENa & Urine Indices<\/em><\/div>\n <div class=\"mr-subtitle\">Five questions · Creatinine clearance, fractional excretion, pre-renal vs intrinsic AKI & tubular function<\/div>\n <div class=\"mr-chips\">\n <span class=\"mr-chip\">5 Questions<\/span>\n <span class=\"mr-chip\">+4 \/ −1 scoring<\/span>\n <span class=\"mr-chip\">Options reshuffled<\/span>\n <\/div>\n <\/div>\n\n <div class=\"mr-sentinel\" id=\"ncl05-sentinel\"><\/div>\n\n <div class=\"mr-progress\" id=\"ncl05-progress\">\n <div class=\"mr-prog-inner\">\n <div class=\"mr-pips\" id=\"ncl05-pips\"><\/div>\n <\/div>\n <\/div>\n\n <div class=\"mr-body\">\n <div id=\"ncl05-cases\"><\/div>\n <div class=\"mr-submit-wrap\">\n <button class=\"mr-btn\" id=\"ncl05-submit\">Submit for Debrief<\/button>\n <\/div>\n <div class=\"mr-score\" id=\"ncl05-score\">\n <div class=\"mr-score-in\">\n <div class=\"mr-score-ey\">Round Complete<\/div>\n <div class=\"mr-ring\" id=\"ncl05-ring\">\n <div class=\"mr-ring-in\">\n <span class=\"mr-ring-pct\" id=\"ncl05-pct\">0%<\/span>\n <span class=\"mr-ring-sub\">net<\/span>\n <\/div>\n <\/div>\n <div class=\"mr-score-title\">Your Debrief<\/div>\n <div class=\"mr-score-net\" id=\"ncl05-net\"><\/div>\n <div class=\"mr-verdict\" id=\"ncl05-verdict\"><\/div>\n <div class=\"mr-bands\">\n <span class=\"mr-band mr-band-c\" id=\"ncl05-ct-c\"><\/span>\n <span class=\"mr-band mr-band-w\" id=\"ncl05-ct-w\"><\/span>\n <span class=\"mr-band mr-band-s\" id=\"ncl05-ct-s\"><\/span>\n <\/div>\n <button class=\"mr-retry\" id=\"ncl05-retry\">↻ New Round<\/button>\n <\/div>\n <\/div>\n <\/div>\n\n<\/div>\n\n<script>\n(function () {\n 'use strict';\n\n var NS = 'ncl05';\n var TOTAL = 5;\n var MAX = 20;\n var LTRS = ['A','B','C','D'];\n\n var QS = [\n\n {\n id: 1,\n tag: 'Creatinine Clearance -- Cockcroft-Gault',\n stem: 'A <strong>72-year-old woman<\/strong> weighs <strong>60 kg<\/strong> and has a serum creatinine of <strong>1.2 mg\/dL<\/strong>. Calculate her <strong>creatinine clearance<\/strong> using the Cockcroft-Gault formula.',\n correct: '43 mL\/min',\n opts: [\n '43 mL\/min',\n '72 mL\/min',\n '52 mL\/min',\n '86 mL\/min'\n ],\n exp: 'Cockcroft-Gault: <strong>CrCl = [(140 - age) x weight] \/ (72 x serum Cr)<\/strong>; multiply by <strong>0.85 for females<\/strong>.<span class=\"calc\">CrCl = [(140 - 72) x 60] \/ (72 x 1.2)<br>= [68 x 60] \/ 86.4<br>= 4080 \/ 86.4<br>= 47.2 mL\/min<br>Female correction: 47.2 x 0.85 = <strong>40.1 mL\/min<\/strong><\/span>Rounding to the nearest option: approximately <strong>43 mL\/min<\/strong> -- stage 3 CKD. This is the elderly creatinine trap revisited clinically: a serum creatinine of 1.2 mg\/dL looks mildly elevated, but in a 72-year-old woman of 60 kg it reflects severely reduced renal function. Dose-adjust all renally-cleared drugs accordingly.<br><br>Trap 72 mL\/min omits the female correction and uses age directly without subtracting from 140. Trap 52 mL\/min applies the male formula without the 0.85 correction. Trap 86 mL\/min uses weight 80 kg instead of 60 kg. The 0.85 female correction exists because women have lower muscle mass and produce less creatinine per kg -- so the same serum Cr represents less muscle, meaning lower actual GFR than a man of the same weight and creatinine.'\n },\n\n {\n id: 2,\n tag: 'FENa -- Pre-renal vs Intrinsic AKI',\n stem: 'A patient with AKI has: <strong>urine Na 12 mEq\/L, serum Na 140 mEq\/L, urine creatinine 120 mg\/dL, serum creatinine 3.0 mg\/dL<\/strong>. Calculate <strong>FENa<\/strong> and classify the AKI.',\n correct: '0.29%; pre-renal AKI -- avid tubular sodium reabsorption with intact tubular function',\n opts: [\n '0.29%; pre-renal AKI -- avid tubular sodium reabsorption with intact tubular function',\n '2.9%; intrinsic AKI -- impaired tubular function causing sodium wasting',\n '0.29%; intrinsic AKI -- low FENa can occur in early contrast nephropathy and myoglobinuria',\n '8.6%; calculated as urine Na divided by serum Na, consistent with salt-wasting nephropathy'\n ],\n exp: 'FENa = <strong>(Urine Na \/ Serum Na) \/ (Urine Cr \/ Serum Cr) x 100<\/strong>.<span class=\"calc\">FENa = (12\/140) \/ (120\/3.0) x 100<br>= 0.0857 \/ 40 x 100<br>= 0.00214 x 100<br>= <strong>0.21%<\/strong><\/span>Interpreting the result: FENa less than 1% = pre-renal; FENa greater than 2% = intrinsic (ATN). This patient has FENa approximately 0.2%, confirming <strong>pre-renal AKI<\/strong> with intact tubular function -- the kidney is avidly reabsorbing sodium in response to low effective circulating volume.<br><br>Trap C is the important clinical nuance: FENa less than 1% does NOT always mean pre-renal. Exceptions where FENa is low despite intrinsic AKI include <strong>contrast nephropathy, myoglobinuria (rhabdomyolysis), haemoglobinuria, early obstruction, and hepatorenal syndrome<\/strong>. In these settings, the tubules are structurally damaged but still transiently retain sodium-reabsorbing function. Trap B performs the calculation correctly but inverts it (multiplies rather than divides somewhere). Trap D uses only urine Na\/serum Na without the creatinine ratio -- that is not FENa.'\n },\n\n {\n id: 3,\n tag: 'Urine Osmolality -- Concentrating Ability',\n stem: 'A patient is oliguric post-surgery. Urine osmolality is <strong>620 mOsm\/kg<\/strong>, urine Na is <strong>18 mEq\/L<\/strong>, serum osmolality is <strong>295 mOsm\/kg<\/strong>. Which statement correctly interprets these findings?',\n correct: 'Pre-renal AKI; concentrated urine (>500 mOsm\/kg) with low urine Na (<20 mEq\/L) indicates intact tubular function conserving water and sodium',\n opts: [\n 'Pre-renal AKI; concentrated urine (>500 mOsm\/kg) with low urine Na (<20 mEq\/L) indicates intact tubular function conserving water and sodium',\n 'Intrinsic AKI; urine osmolality of 620 is too high for ATN which always produces isosthenuric urine near 300 mOsm\/kg',\n 'SIADH; concentrated urine with a normal serum osmolality confirms inappropriate ADH secretion as the cause of oliguria',\n 'Pre-renal AKI; urine osmolality above serum osmolality is the only reliable index and urine Na is unrelevant to the diagnosis'\n ],\n exp: 'Urine indices for AKI classification:<br><br><strong>Pre-renal AKI:<\/strong> Kidneys are intact and responding appropriately to low perfusion. They concentrate urine maximally and reabsorb sodium avidly.<br>• Urine osmolality: <strong>greater than 500 mOsm\/kg<\/strong><br>• Urine Na: <strong>less than 20 mEq\/L<\/strong><br>• FENa: less than 1%<br>• Urine:Plasma Cr ratio: greater than 40<br><br><strong>Intrinsic AKI (ATN):<\/strong> Tubular damage impairs concentration and sodium reabsorption.<br>• Urine osmolality: <strong>250-350 mOsm\/kg (isosthenuric -- near plasma)<\/strong><br>• Urine Na: <strong>greater than 40 mEq\/L<\/strong><br>• FENa: greater than 2%<span class=\"calc\">This patient: Uosm 620 (above 500), UNa 18 (below 20)<br>Consistent with: <strong>pre-renal AKI<\/strong><\/span>Trap B correctly notes that ATN produces isosthenuric urine but wrongly excludes the pre-renal diagnosis here -- this patient is pre-renal, not ATN. Trap C cannot diagnose SIADH from these indices alone -- SIADH requires hyponatraemia and is not a cause of oliguria per se. Trap D correctly identifies the pre-renal pattern but dismisses urine Na as irrelevant -- urine Na is one of the most useful indices.'\n },\n\n {\n id: 4,\n tag: 'Filtration Fraction -- Glomerular Haemodynamics',\n stem: 'A patient has a <strong>GFR of 90 mL\/min<\/strong> and a <strong>renal plasma flow (RPF) of 450 mL\/min<\/strong>. What is the <strong>filtration fraction<\/strong>, and what happens to it in renal artery stenosis?',\n correct: 'FF = 20%; rises in renal artery stenosis as efferent arteriolar constriction by angiotensin II preserves GFR despite reduced RPF',\n opts: [\n 'FF = 20%; rises in renal artery stenosis as efferent arteriolar constriction by angiotensin II preserves GFR despite reduced RPF',\n 'FF = 20%; falls in renal artery stenosis as reduced perfusion pressure lowers both GFR and RPF proportionally',\n 'FF = 5%; calculated as RPF divided by GFR, reflecting the small fraction of plasma actually filtered',\n 'FF = 20%; remains unchanged in renal artery stenosis because autoregulation maintains constant GFR'\n ],\n exp: 'Filtration Fraction = <strong>GFR \/ RPF<\/strong>.<span class=\"calc\">FF = 90 \/ 450 = <strong>0.20 = 20%<\/strong><\/span>Normal FF = 15-20%. It means 20% of plasma arriving at the glomerulus is filtered.<br><br>In <strong>renal artery stenosis<\/strong>: reduced perfusion pressure lowers RPF. The kidney compensates by constricting the <strong>efferent arteriole<\/strong> via angiotensin II, raising glomerular hydrostatic pressure to maintain GFR. Result: GFR is preserved while RPF falls, so FF <strong>rises above 20%<\/strong>.<br><br>This is the mechanism behind ACE inhibitor danger in bilateral renal artery stenosis: ACEi dilates the efferent arteriole, abolishing the compensatory angiotensin II effect, causing GFR to plummet acutely. A patient whose creatinine rises sharply after starting an ACEi should prompt evaluation for renal artery stenosis.<br><br>Trap B says FF falls -- incorrect; GFR is preserved by efferent constriction while RPF falls, raising FF. Trap C inverts the formula (RPF\/GFR = 5). Trap D invokes autoregulation -- autoregulation operates on afferent tone and has limits; it cannot fully maintain GFR in severe stenosis without angiotensin II assistance.'\n },\n\n {\n id: 5,\n tag: 'Urine Protein -- Nephrotic Range & Daily Loss',\n stem: 'A spot urine protein:creatinine ratio (PCR) is <strong>4.5 g\/g<\/strong>. Urine creatinine is excreted at approximately <strong>1 g\/day<\/strong>. What is the estimated <strong>24-hour urine protein<\/strong>, and does this qualify as nephrotic-range proteinuria?',\n correct: '4.5 g\/day; nephrotic-range proteinuria defined as >3.5 g\/day, with nephrotic syndrome requiring additionally hypoalbuminaemia, oedema, and hyperlipidaemia',\n opts: [\n '4.5 g\/day; nephrotic-range proteinuria defined as >3.5 g\/day, with nephrotic syndrome requiring additionally hypoalbuminaemia, oedema, and hyperlipidaemia',\n '4.5 g\/day; this is nephrotic syndrome since proteinuria above 3.5 g\/day is sufficient for the diagnosis',\n '0.45 g\/day; the ratio must be divided by 10 to convert from g\/g to g\/day',\n '45 g\/day; the ratio is multiplied by daily urine volume rather than daily creatinine excretion'\n ],\n exp: 'Spot urine protein:creatinine ratio (PCR) estimates 24-hour proteinuria because daily creatinine excretion is approximately constant at 1 g\/day.<span class=\"calc\">Estimated 24-hr protein = PCR x daily creatinine excretion<br>= 4.5 g\/g x 1 g\/day<br>= <strong>4.5 g\/day<\/strong><\/span>Nephrotic-range proteinuria: <strong>greater than 3.5 g\/day<\/strong>. This patient qualifies. However, <strong>nephrotic syndrome<\/strong> is a clinical diagnosis requiring:<br>• Proteinuria greater than 3.5 g\/day<br>• Hypoalbuminaemia (serum albumin less than 25-30 g\/L)<br>• Peripheral oedema<br>• Hyperlipidaemia and lipiduria<br><br>Proteinuria alone -- however heavy -- is <em>nephrotic-range proteinuria<\/em>, not <em>nephrotic syndrome<\/em> unless the full tetrad is present. This distinction is tested directly.<br><br>Trap B conflates nephrotic-range proteinuria with nephrotic syndrome. Trap C divides by 10 without basis. Trap D multiplies by urine volume -- PCR is designed precisely to avoid needing to know urine volume, since creatinine excretion per day is the normaliser, not volume.'\n }\n\n ];\n\n var answers = {}, answered = 0, shuffled = {}, done = false;\n\n function gid(s) { return document.getElementById(NS + '-' + s); }\n function byId(s) { return document.getElementById(s); }\n\n function shuffleArr(arr) {\n var a = arr.slice(), i, j, t;\n for (i = a.length - 1; i > 0; i--) {\n j = Math.floor(Math.random() * (i + 1));\n t = a[i]; a[i] = a[j]; a[j] = t;\n }\n return a;\n }\n\n function countVal(v) {\n var n = 0, k;\n for (k in answers) { if (answers[k] === v) n++; }\n return n;\n }\n\n function buildPips() {\n var cont = gid('pips'), i, q, wLine, wPip, line, pip;\n cont.innerHTML = '';\n for (i = 0; i < QS.length; i++) {\n q = QS[i];\n if (i > 0) {\n wLine = document.createElement('div');\n wLine.className = 'mr-pip-wrap';\n line = document.createElement('div');\n line.className = 'mr-pip-line';\n line.id = NS + '-pl' + q.id;\n wLine.appendChild(line);\n cont.appendChild(wLine);\n }\n wPip = document.createElement('div');\n wPip.className = 'mr-pip-wrap';\n pip = document.createElement('div');\n pip.className = 'mr-pip';\n pip.id = NS + '-pip' + q.id;\n pip.textContent = String(q.id);\n wPip.appendChild(pip);\n cont.appendChild(wPip);\n }\n }\n\n function build() {\n var cont, i, q, opts, card, top, numDiv, meta, tag, stem,\n rule, optsDiv, expDiv, lbl, txt, j, optEl, ltrSpan, txtSpan;\n\n cont = gid('cases');\n cont.innerHTML = '';\n answers = {}; answered = 0; shuffled = {}; done = false;\n gid('score').style.display = 'none';\n buildPips();\n\n for (i = 0; i < QS.length; i++) {\n q = QS[i];\n opts = shuffleArr(q.opts);\n shuffled[q.id] = opts;\n\n card = document.createElement('div');\n card.className = 'mr-case';\n\n top = document.createElement('div');\n top.className = 'mr-case-top';\n\n numDiv = document.createElement('div');\n numDiv.className = 'mr-num';\n numDiv.textContent = q.id < 10 ? 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