{"id":37027,"date":"2026-06-11T20:38:37","date_gmt":"2026-06-11T15:08:37","guid":{"rendered":"https:\/\/atsixty.com\/?p=37027"},"modified":"2026-06-11T20:44:45","modified_gmt":"2026-06-11T15:14:45","slug":"renal-tubular-disorders","status":"publish","type":"post","link":"https:\/\/atsixty.com\/index.php\/morning-rounds\/renal-tubular-disorders\/","title":{"rendered":"Renal Tubular Disorders"},"content":{"rendered":"\n\n\n<meta charset=\"UTF-8\">\n<meta name=\"viewport\" content=\"width=device-width, initial-scale=1.0\">\n<title>Morning Rounds \u00b7 Renal Tubular Disorders<\/title>\n<link href=\"https:\/\/fonts.googleapis.com\/css2?family=Playfair+Display:ital,wght@0,400;0,600;0,700;1,400;1,600&#038;family=Source+Serif+4:ital,wght@0,300;0,400;0,600;1,400&#038;display=swap\" rel=\"stylesheet\">\n<style>\n#nep04 *,#nep04 *::before,#nep04 *::after{box-sizing:border-box;margin:0;padding:0}\n#nep04{\n  --nep:#2A5470;--nep-light:#3A7499;--nep-pale:#EBF3F8;--nep-dark:#1C3D52;\n  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font-weight=\"bold\">Renal Tubular Acidosis \u2014 Comparison<\/text>\n      <!-- Headers -->\n      <rect x=\"10\"  y=\"22\" width=\"100\" height=\"20\" rx=\"3\" fill=\"#2A5470\"\/>\n      <rect x=\"114\" y=\"22\" width=\"100\" height=\"20\" rx=\"3\" fill=\"#2A5470\"\/>\n      <rect x=\"218\" y=\"22\" width=\"100\" height=\"20\" rx=\"3\" fill=\"#2A5470\"\/>\n      <rect x=\"322\" y=\"22\" width=\"100\" height=\"20\" rx=\"3\" fill=\"#2A5470\"\/>\n      <rect x=\"426\" y=\"22\" width=\"124\" height=\"20\" rx=\"3\" fill=\"#2A5470\"\/>\n      <text x=\"60\"  y=\"35\" text-anchor=\"middle\" fill=\"#F0F7FC\" font-size=\"8\" font-family=\"Georgia,serif\" font-weight=\"bold\">Feature<\/text>\n      <text x=\"164\" y=\"35\" text-anchor=\"middle\" fill=\"#F0F7FC\" font-size=\"8\" font-family=\"Georgia,serif\" font-weight=\"bold\">Type I (Distal)<\/text>\n      <text x=\"268\" y=\"35\" text-anchor=\"middle\" fill=\"#F0F7FC\" font-size=\"8\" font-family=\"Georgia,serif\" font-weight=\"bold\">Type II (Proximal)<\/text>\n      <text x=\"372\" y=\"35\" text-anchor=\"middle\" fill=\"#F0F7FC\" font-size=\"8\" font-family=\"Georgia,serif\" font-weight=\"bold\">Type IV<\/text>\n      <text x=\"488\" y=\"35\" text-anchor=\"middle\" fill=\"#F0F7FC\" font-size=\"8\" font-family=\"Georgia,serif\" font-weight=\"bold\">Normal AG acidosis<\/text>\n      <!-- Row: Defect -->\n      <rect x=\"10\"  y=\"44\" width=\"100\" height=\"22\" rx=\"2\" fill=\"#d4e8f4\"\/>\n      <rect x=\"114\" y=\"44\" width=\"100\" height=\"22\" rx=\"2\" fill=\"#e8f3f9\"\/>\n      <rect x=\"218\" y=\"44\" width=\"100\" height=\"22\" rx=\"2\" fill=\"#e8f3f9\"\/>\n      <rect x=\"322\" y=\"44\" width=\"100\" height=\"22\" rx=\"2\" fill=\"#e8f3f9\"\/>\n      <rect x=\"426\" y=\"44\" width=\"124\" height=\"22\" rx=\"2\" fill=\"#f4f9fc\"\/>\n      <text x=\"60\"  y=\"54\" text-anchor=\"middle\" fill=\"#1C3D52\" font-size=\"7.5\" font-family=\"Georgia,serif\">Defect<\/text>\n      <text x=\"164\" y=\"53\" text-anchor=\"middle\" fill=\"#143a4e\" font-size=\"7\" font-family=\"Georgia,serif\">H&#x207A; secretion<\/text>\n      <text x=\"164\" y=\"63\" text-anchor=\"middle\" fill=\"#143a4e\" font-size=\"7\" font-family=\"Georgia,serif\">(collecting duct)<\/text>\n      <text x=\"268\" y=\"53\" text-anchor=\"middle\" fill=\"#143a4e\" font-size=\"7\" font-family=\"Georgia,serif\">HCO&#x2083;&#x207B; reabsorption<\/text>\n      <text x=\"268\" y=\"63\" text-anchor=\"middle\" fill=\"#143a4e\" font-size=\"7\" font-family=\"Georgia,serif\">(proximal tubule)<\/text>\n      <text x=\"372\" y=\"53\" text-anchor=\"middle\" fill=\"#143a4e\" font-size=\"7\" font-family=\"Georgia,serif\">Aldosterone deficiency<\/text>\n      <text x=\"372\" y=\"63\" text-anchor=\"middle\" fill=\"#143a4e\" font-size=\"7\" font-family=\"Georgia,serif\">\/ resistance<\/text>\n      <text x=\"488\" y=\"54\" text-anchor=\"middle\" fill=\"#7a9bad\" font-size=\"7\" font-family=\"Georgia,serif\" font-style=\"italic\">All three are NAGMA<\/text>\n      <!-- Row: Serum K -->\n      <rect x=\"10\"  y=\"68\" width=\"100\" height=\"20\" rx=\"2\" fill=\"#c4dff0\"\/>\n      <rect x=\"114\" y=\"68\" width=\"100\" height=\"20\" rx=\"2\" fill=\"#fdf0f0\"\/>\n      <rect x=\"218\" y=\"68\" width=\"100\" height=\"20\" rx=\"2\" fill=\"#fdf0f0\"\/>\n      <rect x=\"322\" y=\"68\" width=\"100\" height=\"20\" rx=\"2\" fill=\"#eaf6ef\"\/>\n      <rect x=\"426\" y=\"68\" width=\"124\" height=\"20\" rx=\"2\" fill=\"#f4f9fc\"\/>\n      <text x=\"60\"  y=\"81\" text-anchor=\"middle\" fill=\"#1C3D52\" font-size=\"7.5\" font-family=\"Georgia,serif\">Serum K&#x207A;<\/text>\n      <text x=\"164\" y=\"81\" text-anchor=\"middle\" fill=\"#B83232\" font-size=\"8\" font-family=\"Georgia,serif\" font-weight=\"bold\">&#x2193; Low<\/text>\n      <text x=\"268\" y=\"81\" text-anchor=\"middle\" fill=\"#B83232\" font-size=\"8\" font-family=\"Georgia,serif\" font-weight=\"bold\">&#x2193; Low<\/text>\n      <text x=\"372\" y=\"81\" text-anchor=\"middle\" fill=\"#2D6B47\" font-size=\"8\" font-family=\"Georgia,serif\" font-weight=\"bold\">&#x2191; High<\/text>\n      <text x=\"488\" y=\"81\" text-anchor=\"middle\" fill=\"#7a9bad\" font-size=\"7\" font-family=\"Georgia,serif\" font-style=\"italic\">Key distinguishing point<\/text>\n      <!-- Row: Urine pH -->\n      <rect x=\"10\"  y=\"90\" width=\"100\" height=\"20\" rx=\"2\" fill=\"#d4e8f4\"\/>\n      <rect x=\"114\" y=\"90\" width=\"100\" height=\"20\" rx=\"2\" fill=\"#e8f3f9\"\/>\n      <rect x=\"218\" y=\"90\" width=\"100\" height=\"20\" rx=\"2\" fill=\"#e8f3f9\"\/>\n      <rect x=\"322\" y=\"90\" width=\"100\" height=\"20\" rx=\"2\" fill=\"#e8f3f9\"\/>\n      <rect x=\"426\" y=\"90\" width=\"124\" height=\"20\" rx=\"2\" fill=\"#f4f9fc\"\/>\n      <text x=\"60\"  y=\"103\" text-anchor=\"middle\" fill=\"#1C3D52\" font-size=\"7.5\" font-family=\"Georgia,serif\">Urine pH<\/text>\n      <text x=\"164\" y=\"103\" text-anchor=\"middle\" fill=\"#143a4e\" font-size=\"8\" font-family=\"Georgia,serif\" font-weight=\"bold\">&gt; 5.5 (cannot acidify)<\/text>\n      <text x=\"268\" y=\"103\" text-anchor=\"middle\" fill=\"#143a4e\" font-size=\"7.5\" font-family=\"Georgia,serif\">Variable \/ &lt;5.5 possible<\/text>\n      <text x=\"372\" y=\"103\" text-anchor=\"middle\" fill=\"#143a4e\" font-size=\"7.5\" font-family=\"Georgia,serif\">&#x2264;5.5<\/text>\n      <text x=\"488\" y=\"103\" text-anchor=\"middle\" fill=\"#7a9bad\" font-size=\"7\" font-family=\"Georgia,serif\" font-style=\"italic\">Type I: always &gt;5.5<\/text>\n      <!-- Row: Stones\/nephrocalcinosis -->\n      <rect x=\"10\"  y=\"112\" width=\"100\" height=\"20\" rx=\"2\" fill=\"#c4dff0\"\/>\n      <rect x=\"114\" y=\"112\" width=\"100\" height=\"20\" rx=\"2\" fill=\"#fff5e0\"\/>\n      <rect x=\"218\" y=\"112\" width=\"100\" height=\"20\" rx=\"2\" fill=\"#e8f3f9\"\/>\n      <rect x=\"322\" y=\"112\" width=\"100\" height=\"20\" rx=\"2\" fill=\"#e8f3f9\"\/>\n      <rect x=\"426\" y=\"112\" width=\"124\" height=\"20\" rx=\"2\" fill=\"#f4f9fc\"\/>\n      <text x=\"60\"  y=\"125\" text-anchor=\"middle\" fill=\"#1C3D52\" font-size=\"7.5\" font-family=\"Georgia,serif\">Stones \/ NCC<\/text>\n      <text x=\"164\" y=\"125\" text-anchor=\"middle\" fill=\"#7a5000\" font-size=\"8\" font-family=\"Georgia,serif\" font-weight=\"bold\">Yes (nephrocalcinosis)<\/text>\n      <text x=\"268\" y=\"125\" text-anchor=\"middle\" fill=\"#143a4e\" font-size=\"7.5\" font-family=\"Georgia,serif\">Rare<\/text>\n      <text x=\"372\" y=\"125\" text-anchor=\"middle\" fill=\"#143a4e\" font-size=\"7.5\" font-family=\"Georgia,serif\">No<\/text>\n      <text x=\"488\" y=\"125\" text-anchor=\"middle\" fill=\"#7a9bad\" font-size=\"7\" font-family=\"Georgia,serif\" font-style=\"italic\">Classic Type I feature<\/text>\n      <!-- Row: Treatment -->\n      <rect x=\"10\"  y=\"134\" width=\"100\" height=\"22\" rx=\"2\" fill=\"#d4e8f4\"\/>\n      <rect x=\"114\" y=\"134\" width=\"100\" height=\"22\" rx=\"2\" fill=\"#eaf6ef\"\/>\n      <rect x=\"218\" y=\"134\" width=\"100\" height=\"22\" rx=\"2\" fill=\"#eaf6ef\"\/>\n      <rect x=\"322\" y=\"134\" width=\"100\" height=\"22\" rx=\"2\" fill=\"#eaf6ef\"\/>\n      <rect x=\"426\" y=\"134\" width=\"124\" height=\"22\" rx=\"2\" fill=\"#f4f9fc\"\/>\n      <text x=\"60\"  y=\"144\" text-anchor=\"middle\" fill=\"#1C3D52\" font-size=\"7.5\" font-family=\"Georgia,serif\">Treatment<\/text>\n      <text x=\"164\" y=\"143\" text-anchor=\"middle\" fill=\"#2D6B47\" font-size=\"7\" font-family=\"Georgia,serif\">Oral NaHCO&#x2083;<\/text>\n      <text x=\"164\" y=\"153\" text-anchor=\"middle\" fill=\"#2D6B47\" font-size=\"7\" font-family=\"Georgia,serif\">or K-citrate<\/text>\n      <text x=\"268\" y=\"143\" text-anchor=\"middle\" fill=\"#2D6B47\" font-size=\"7\" font-family=\"Georgia,serif\">Large doses NaHCO&#x2083;<\/text>\n      <text x=\"268\" y=\"153\" text-anchor=\"middle\" fill=\"#2D6B47\" font-size=\"7\" font-family=\"Georgia,serif\">+ K supplementation<\/text>\n      <text x=\"372\" y=\"143\" text-anchor=\"middle\" fill=\"#2D6B47\" font-size=\"7\" font-family=\"Georgia,serif\">Fludrocortisone<\/text>\n      <text x=\"372\" y=\"153\" text-anchor=\"middle\" fill=\"#2D6B47\" font-size=\"7\" font-family=\"Georgia,serif\">or loop diuretic<\/text>\n      <text x=\"488\" y=\"144\" text-anchor=\"middle\" fill=\"#7a9bad\" font-size=\"7\" font-family=\"Georgia,serif\" font-style=\"italic\">Treat cause + alkali<\/text>\n      <!-- Row: Causes -->\n      <rect x=\"10\"  y=\"158\" width=\"100\" height=\"36\" rx=\"2\" fill=\"#c4dff0\"\/>\n      <rect x=\"114\" y=\"158\" width=\"100\" height=\"36\" rx=\"2\" fill=\"#e8f3f9\"\/>\n      <rect x=\"218\" y=\"158\" width=\"100\" height=\"36\" rx=\"2\" fill=\"#e8f3f9\"\/>\n      <rect x=\"322\" y=\"158\" width=\"100\" height=\"36\" rx=\"2\" fill=\"#e8f3f9\"\/>\n      <rect x=\"426\" y=\"158\" width=\"124\" height=\"36\" rx=\"2\" fill=\"#f4f9fc\"\/>\n      <text x=\"60\"  y=\"170\" text-anchor=\"middle\" fill=\"#1C3D52\" font-size=\"7.5\" font-family=\"Georgia,serif\">Causes<\/text>\n      <text x=\"164\" y=\"168\" text-anchor=\"middle\" fill=\"#143a4e\" font-size=\"7\" font-family=\"Georgia,serif\">Sj&#xF6;gren, SLE,<\/text>\n      <text x=\"164\" y=\"178\" text-anchor=\"middle\" fill=\"#143a4e\" font-size=\"7\" font-family=\"Georgia,serif\">amphotericin B,<\/text>\n      <text x=\"164\" y=\"188\" text-anchor=\"middle\" fill=\"#143a4e\" font-size=\"7\" font-family=\"Georgia,serif\">lithium<\/text>\n      <text x=\"268\" y=\"168\" text-anchor=\"middle\" fill=\"#143a4e\" font-size=\"7\" font-family=\"Georgia,serif\">Fanconi (Wilson,<\/text>\n      <text x=\"268\" y=\"178\" text-anchor=\"middle\" fill=\"#143a4e\" font-size=\"7\" font-family=\"Georgia,serif\">cystinosis, tenofovir,<\/text>\n      <text x=\"268\" y=\"188\" text-anchor=\"middle\" fill=\"#143a4e\" font-size=\"7\" font-family=\"Georgia,serif\">multiple myeloma)<\/text>\n      <text x=\"372\" y=\"168\" text-anchor=\"middle\" fill=\"#143a4e\" font-size=\"7\" font-family=\"Georgia,serif\">DM, hyporeninism,<\/text>\n      <text x=\"372\" y=\"178\" text-anchor=\"middle\" fill=\"#143a4e\" font-size=\"7\" font-family=\"Georgia,serif\">ACEi\/ARB,<\/text>\n      <text x=\"372\" y=\"188\" text-anchor=\"middle\" fill=\"#143a4e\" font-size=\"7\" font-family=\"Georgia,serif\">heparin, CKD<\/text>\n      <text x=\"488\" y=\"168\" text-anchor=\"middle\" fill=\"#7a9bad\" font-size=\"7\" font-family=\"Georgia,serif\" font-style=\"italic\">No Type III in<\/text>\n      <text x=\"488\" y=\"178\" text-anchor=\"middle\" fill=\"#7a9bad\" font-size=\"7\" font-family=\"Georgia,serif\" font-style=\"italic\">current classification<\/text>\n      <text x=\"488\" y=\"188\" text-anchor=\"middle\" fill=\"#7a9bad\" font-size=\"7\" font-family=\"Georgia,serif\" font-style=\"italic\">(obsolete)<\/text>\n    <\/svg>\n    <figcaption>\n      All three RTA types produce a <strong>normal anion gap metabolic acidosis (NAGMA)<\/strong>. The serum potassium is the pivotal differentiator: <em>low<\/em> in Types I and II, <em>high<\/em> in Type IV. Urine pH &gt;5.5 in the face of systemic acidosis is pathognomonic of Type I \u2014 the distal tubule cannot secrete H\u207a.\n    <\/figcaption>\n  <\/figure>\n<\/div>\n\n<div id=\"nep04\">\n  <div class=\"mr-header\">\n    <div class=\"mr-eyebrow\">Morning Rounds \u00b7 Nephrology Series \u00b7 Round 04<\/div>\n    <div class=\"mr-title\">Renal Tubular Disorders<br><em>RTA, Fanconi &amp; Tubular Physiology<\/em><\/div>\n    <div class=\"mr-subtitle\">Five cases &middot; Read carefully &middot; Trust your instinct<\/div>\n    <div class=\"mr-chips\">\n      <span class=\"mr-chip\">5 Cases<\/span>\n      <span class=\"mr-chip\">+4 \/ &minus;1 scoring<\/span>\n      <span class=\"mr-chip\">Options reshuffled<\/span>\n    <\/div>\n  <\/div>\n\n  <div class=\"mr-sentinel\" id=\"nep04-sentinel\"><\/div>\n\n  <div class=\"mr-progress\" id=\"nep04-progress\">\n    <div class=\"mr-prog-inner\">\n      <div class=\"mr-pips\" id=\"nep04-pips\"><\/div>\n    <\/div>\n  <\/div>\n\n  <div class=\"mr-body\">\n    <div id=\"nep04-cases\"><\/div>\n    <div class=\"mr-submit-wrap\">\n      <button class=\"mr-btn\" id=\"nep04-submit\">Submit for Debrief<\/button>\n    <\/div>\n    <div class=\"mr-score\" id=\"nep04-score\">\n      <div class=\"mr-score-in\">\n        <div class=\"mr-score-ey\">Round Complete<\/div>\n        <div class=\"mr-ring\" id=\"nep04-ring\">\n          <div class=\"mr-ring-in\">\n            <span class=\"mr-ring-pct\" id=\"nep04-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=\"nep04-net\"><\/div>\n        <div class=\"mr-verdict\" id=\"nep04-verdict\"><\/div>\n        <div class=\"mr-bands\">\n          <span class=\"mr-band mr-band-c\" id=\"nep04-ct-c\"><\/span>\n          <span class=\"mr-band mr-band-w\" id=\"nep04-ct-w\"><\/span>\n          <span class=\"mr-band mr-band-s\" id=\"nep04-ct-s\"><\/span>\n        <\/div>\n        <button class=\"mr-retry\" id=\"nep04-retry\">&#8635; New Round<\/button>\n      <\/div>\n    <\/div>\n  <\/div>\n<\/div><!-- end #nep04 -->\n\n<script>\n(function(){\n  'use strict';\n  var NS='nep04',TOTAL=5,MAX=20,LTRS=['A','B','C','D'];\n\n  var QS=[\n    {\n      id:1,\n      tag:'Renal Tubular Acidosis &mdash; Type I vs Type IV',\n      stem:'A <strong>38-year-old woman<\/strong> with Sj&ouml;gren syndrome presents with muscle weakness and cramps. Investigations: serum Na 138, K <strong>2.8 mEq\/L<\/strong>, Cl 112, HCO&#x2083;&#x207B; <strong>13 mEq\/L<\/strong>, creatinine 0.9 mg\/dL. Arterial blood gas: pH 7.28, pCO&#x2082; 30 mmHg (appropriate compensation). Urine pH <strong>6.8<\/strong> despite systemic acidosis. X-ray shows <strong>medullary nephrocalcinosis<\/strong>. What is the diagnosis and why is urine pH the decisive clue?',\n      correct:'Type I (distal) RTA; the distal tubule cannot acidify urine &mdash; pH remains &gt;5.5 even in systemic acidosis',\n      opts:[\n        'Type I (distal) RTA; the distal tubule cannot acidify urine &mdash; pH remains &gt;5.5 even in systemic acidosis',\n        'Type IV RTA; hyperkalaemia and aldosterone deficiency are the defining features',\n        'Type II (proximal) RTA; bicarbonate wasting leads to urine pH &gt;5.5 only when serum HCO&#x2083;&#x207B; is above threshold',\n        'Diarrhoea-induced NAGMA; stool bicarbonate loss explains both acidosis and hypokalaemia'\n      ],\n      exp:'<strong>Type I (distal) RTA<\/strong> results from failure of the collecting duct to secrete H&#x207A;, so urine <em>cannot<\/em> be acidified below pH 5.5 regardless of how acidaemic the patient is. The serum potassium is <strong>low<\/strong> (hypokalemia) because urinary H&#x207A; retention is replaced by K&#x207A; loss. The anion gap is normal: AG = 138 &minus; (112 + 13) = 13 &mdash; borderline, consistent with NAGMA. <strong>Nephrocalcinosis and nephrolithiasis<\/strong> (calcium phosphate stones) are hallmarks of Type I due to hypercalciuria, hypocitraturia, and persistently alkaline urine favouring calcium precipitation. Sj&ouml;gren syndrome is a classic autoimmune cause. <strong>Type IV<\/strong> would show <em>hyperkalaemia<\/em> \u2014 the opposite. Type II loses HCO&#x2083;&#x207B; only until serum HCO&#x2083;&#x207B; falls below the reabsorption threshold, after which urine <em>can<\/em> acidify. Treatment: oral potassium citrate or sodium bicarbonate.',\n      imgId:'nep04-img1'\n    },\n    {\n      id:2,\n      tag:'Renal Tubular Disorders &mdash; Fanconi Syndrome',\n      stem:'A <strong>30-year-old HIV-positive man<\/strong> on tenofovir disoproxil fumarate (TDF) for 3 years develops bone pain and proximal muscle weakness. Investigations: serum phosphate <strong>1.8 mg\/dL<\/strong> (low), uric acid <strong>2.1 mg\/dL<\/strong> (low), potassium 3.1 mEq\/L, bicarbonate 17 mEq\/L, glucose 96 mg\/dL. Urine shows <strong>glycosuria<\/strong>, <strong>phosphaturia<\/strong>, and <strong>aminoaciduria<\/strong> with serum glucose normal. What is the diagnosis?',\n      correct:'Fanconi syndrome secondary to tenofovir nephrotoxicity; generalised proximal tubular dysfunction',\n      opts:[\n        'Fanconi syndrome secondary to tenofovir nephrotoxicity; generalised proximal tubular dysfunction',\n        'Type I RTA secondary to TDF; selective distal acidification defect',\n        'Hypophosphataemic rickets; isolated phosphate wasting without other tubular defects',\n        'Diabetic nephropathy with glycosuria; SGLT2-like effect of TDF on proximal tubule'\n      ],\n      exp:'<strong>Fanconi syndrome<\/strong> is a <em>generalised<\/em> proximal tubular dysfunction causing urinary wasting of <strong>everything the proximal tubule normally reabsorbs<\/strong>: glucose (despite normoglycaemia), phosphate, uric acid, amino acids, potassium, bicarbonate, and sometimes sodium. The mnemonic: <strong>GARBAGE<\/strong> \u2014 Glucose, Amino acids, RBC precursor phosphate, Bicarbonate, Aminoaciduria, Glycosuria, Electrolytes. <strong>Tenofovir (TDF)<\/strong> is a well-recognised cause via mitochondrial toxicity of proximal tubular cells \u2014 it inhibits mitochondrial DNA polymerase gamma. Other causes: cystinosis (children), Wilson disease, multiple myeloma (light chain toxicity), ifosfamide, lead. The bone pain reflects <strong>osteomalacia<\/strong> from phosphate wasting. Management: switch to TAF (tenofovir alafenamide, less nephrotoxic) or an alternative ARV; phosphate supplementation; vitamin D.',\n      imgId:null\n    },\n    {\n      id:3,\n      tag:'Renal Tubular Disorders &mdash; Bartter vs Gitelman',\n      stem:'Two patients present with similar metabolic profiles: both have <strong>hypokalaemic metabolic alkalosis<\/strong> with normal blood pressure and normal-to-low serum magnesium. Patient A is a <strong>6-month-old<\/strong> with polyuria, failure to thrive, and hypercalciuria. Patient B is a <strong>35-year-old woman<\/strong> detected incidentally with low potassium, <strong>hypomagnesaemia<\/strong>, and <strong>hypocalciuria<\/strong>. What is the correct diagnosis for each?',\n      correct:'Patient A: Bartter syndrome; Patient B: Gitelman syndrome',\n      opts:[\n        'Patient A: Bartter syndrome; Patient B: Gitelman syndrome',\n        'Patient A: Gitelman syndrome; Patient B: Bartter syndrome',\n        'Both: Bartter syndrome &mdash; Gitelman is a variant seen only in adults over 50',\n        'Patient A: SIADH with pseudo-hypokalaemia; Patient B: primary hyperaldosteronism'\n      ],\n      exp:'Both Bartter and Gitelman mimic chronic loop or thiazide diuretic use respectively \u2014 both cause hypokalaemic metabolic alkalosis with normal\/low BP and high renin\/aldosterone. The key differences: <strong>Bartter syndrome<\/strong> \u2014 defect in the <em>thick ascending limb<\/em> (NKCC2 or ROMK channels, mimics furosemide); presents in <em>infancy or childhood<\/em>; associated with <strong>hypercalciuria and nephrocalcinosis<\/strong>, polyuria, polydipsia, growth retardation. <strong>Gitelman syndrome<\/strong> \u2014 defect in the <em>distal convoluted tubule<\/em> (NCC thiazide-sensitive cotransporter, mimics thiazides); presents in <em>older children or adults<\/em>, often incidentally; characterised by <strong>hypomagnesaemia and hypocalciuria<\/strong> (calcium is avidly reabsorbed). The calcium distinction is the exam pivot: <em>Bartter = hypercalciuria; Gitelman = hypocalciuria<\/em>. Treatment: both use potassium and magnesium supplementation; NSAIDs (indomethacin) for Bartter in infancy.',\n      imgId:null\n    },\n    {\n      id:4,\n      tag:'Renal Tubular Disorders &mdash; Type IV RTA',\n      stem:'A <strong>62-year-old man<\/strong> with type 2 diabetes and CKD G3a (eGFR 48) is started on an ACE inhibitor for proteinuria. At follow-up: serum K <strong>6.2 mEq\/L<\/strong>, bicarbonate <strong>19 mEq\/L<\/strong>, pH 7.32, urine pH 5.1, creatinine stable at 1.6 mg\/dL. There is no dietary excess. What is the mechanism of his hyperkalaemic metabolic acidosis?',\n      correct:'Type IV RTA; hyporeninism-hypoaldosteronism in diabetic nephropathy, exacerbated by ACE inhibitor',\n      opts:[\n        'Type IV RTA; hyporeninism-hypoaldosteronism in diabetic nephropathy, exacerbated by ACE inhibitor',\n        'Type I RTA; urine pH &lt;5.5 excludes distal acidification defect',\n        'AKI from ACE inhibitor; creatinine should be rechecked urgently',\n        'Pseudohyperkalaemia; repeat sample with immediate processing'\n      ],\n      exp:'<strong>Type IV RTA<\/strong> is the commonest RTA in adults and is almost always due to <strong>hypoaldosteronism or aldosterone resistance<\/strong>. In diabetic nephropathy, destruction of the juxtaglomerular apparatus causes <strong>hyporeninism &#x2192; low angiotensin II &#x2192; low aldosterone<\/strong>. Without aldosterone, the collecting duct cannot secrete K&#x207A; or H&#x207A; &#x2192; <strong>hyperkalaemia + mild metabolic acidosis<\/strong>. <strong>ACE inhibitors<\/strong> further suppress angiotensin II &#x2192; aldosterone, compounding the effect. The urine pH is <strong>&#x2264;5.5<\/strong> (here 5.1) \u2014 the distal tubule <em>can<\/em> secrete H&#x207A;, it just cannot do so enough because aldosterone is absent (aldosterone normally upregulates H&#x207A;-ATPase). This distinguishes it from Type I (urine pH always &gt;5.5). Other drugs causing Type IV RTA: heparin, NSAIDs, calcineurin inhibitors, trimethoprim. Management: reduce\/stop ACE inhibitor if K&gt;6.5; fludrocortisone if hyporeninism; low-potassium diet; sodium bicarbonate.',\n      imgId:null\n    },\n    {\n      id:5,\n      tag:'Renal Tubular Disorders &mdash; Nephrogenic Diabetes Insipidus',\n      stem:'A <strong>45-year-old man<\/strong> on long-term <strong>lithium<\/strong> therapy for bipolar disorder presents with polyuria (6\u20138 litres\/day) and polydipsia. Serum sodium is <strong>148 mEq\/L<\/strong>, serum osmolality <strong>304 mOsm\/kg<\/strong>, urine osmolality <strong>120 mOsm\/kg<\/strong>. After <strong>desmopressin (DDAVP)<\/strong> administration, urine osmolality rises to only <strong>135 mOsm\/kg<\/strong> (minimal response). What is the diagnosis and mechanism?',\n      correct:'Nephrogenic diabetes insipidus; lithium blocks ADH-mediated aquaporin-2 insertion in collecting duct principal cells',\n      opts:[\n        'Nephrogenic diabetes insipidus; lithium blocks ADH-mediated aquaporin-2 insertion in collecting duct principal cells',\n        'Central diabetes insipidus; lithium suppresses ADH secretion from the posterior pituitary',\n        'Primary polydipsia; serum sodium would be low, not high, in compulsive water drinkers',\n        'SIADH; inappropriately low urine osmolality confirms ADH excess'\n      ],\n      exp:'<strong>Nephrogenic DI (NDI)<\/strong> occurs when the kidney fails to respond to ADH (vasopressin), even when ADH levels are adequate or high. <strong>Lithium<\/strong> is the commonest drug cause \u2014 it enters principal cells of the collecting duct via ENaC channels and accumulates intracellularly, <strong>inhibiting adenylyl cyclase<\/strong> downstream of the V2 receptor, thereby preventing cAMP-mediated <strong>aquaporin-2 (AQP2)<\/strong> channel insertion into the apical membrane. Without AQP2, the tubule is impermeable to water regardless of ADH levels. The desmopressin test distinguishes: <strong>central DI<\/strong> &#x2192; brisk rise in urine osmolality after DDAVP (&gt;50% rise or &gt;750 mOsm\/kg); <strong>nephrogenic DI<\/strong> &#x2192; minimal or no response (as here). Hypernatraemia + dilute urine + high serum osmolality = DI. Primary polydipsia gives <em>low<\/em> serum Na (dilutional). Management of lithium-NDI: thiazide diuretics (paradoxically reduce urine output by causing mild volume depletion, increasing proximal reabsorption), amiloride (blocks ENaC, reducing lithium entry), lowest effective lithium dose.',\n      imgId:null\n    }\n  ];\n\n  var answers={},answered=0,shuffled={},done=false;\n  function byId(id){return document.getElementById(id);}\n  function gid(s){return byId(NS+'-'+s);}\n  function shuffleArr(arr){\n    var a=arr.slice(),i,j,tmp;\n    for(i=a.length-1;i>0;i--){j=Math.floor(Math.random()*(i+1));tmp=a[i];a[i]=a[j];a[j]=tmp;}\n    return a;\n  }\n  function countVal(val){var k,n=0;for(k in answers){if(answers.hasOwnProperty(k)&&answers[k]===val)n++;}return 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){wLine=document.createElement('div');wLine.className='mr-pip-wrap';line=document.createElement('div');line.className='mr-pip-line';line.id=NS+'-pl'+q.id;wLine.appendChild(line);cont.appendChild(wLine);}\n      wPip=document.createElement('div');wPip.className='mr-pip-wrap';pip=document.createElement('div');pip.className='mr-pip';pip.id=NS+'-pip'+q.id;pip.textContent=String(q.id);wPip.appendChild(pip);cont.appendChild(wPip);\n    }\n  }\n\n  function build(){\n    var cont,i,q,opts,card,top,numDiv,meta,tag,stem,rule,optsDiv,expDiv,lbl,txt,imgDiv,imgSrc,j,optEl,ltrSpan,txtSpan;\n    cont=gid('cases');cont.innerHTML='';\n    answers={};answered=0;shuffled={};done=false;\n    gid('score').style.display='none';\n    buildPips();\n    for(i=0;i<QS.length;i++){\n      q=QS[i];opts=shuffleArr(q.opts);shuffled[q.id]=opts;\n      card=document.createElement('div');card.className='mr-case';\n      top=document.createElement('div');top.className='mr-case-top';\n      numDiv=document.createElement('div');numDiv.className='mr-num';numDiv.textContent=q.id<10?'0'+q.id:String(q.id);\n      meta=document.createElement('div');meta.className='mr-meta';\n      tag=document.createElement('div');tag.className='mr-tag';tag.innerHTML=q.tag;\n      stem=document.createElement('div');stem.className='mr-stem';stem.innerHTML=q.stem;\n      meta.appendChild(tag);meta.appendChild(stem);top.appendChild(numDiv);top.appendChild(meta);card.appendChild(top);\n      rule=document.createElement('div');rule.className='mr-rule';card.appendChild(rule);\n      optsDiv=document.createElement('div');optsDiv.className='mr-opts';\n      for(j=0;j<opts.length;j++){\n        optEl=document.createElement('div');optEl.className='mr-opt';optEl.id=NS+'-o'+q.id+'-'+j;optEl.setAttribute('role','button');optEl.setAttribute('tabindex','0');\n        ltrSpan=document.createElement('span');ltrSpan.className='mr-ltr';ltrSpan.textContent=LTRS[j];\n        txtSpan=document.createElement('span');txtSpan.className='mr-opt-text';txtSpan.innerHTML=opts[j];\n        optEl.appendChild(ltrSpan);optEl.appendChild(txtSpan);optsDiv.appendChild(optEl);\n        (function(qid,oi){optEl.addEventListener('click',function(){pick(qid,oi);});}(q.id,j));\n      }\n      card.appendChild(optsDiv);\n      expDiv=document.createElement('div');expDiv.className='mr-exp';expDiv.id=NS+'-exp'+q.id;\n      lbl=document.createElement('div');lbl.className='mr-exp-lbl';lbl.textContent='Debrief';\n      txt=document.createElement('div');txt.className='mr-exp-text';txt.innerHTML=q.exp;\n      expDiv.appendChild(lbl);expDiv.appendChild(txt);\n      if(q.imgId){imgSrc=byId(q.imgId);if(imgSrc){imgDiv=document.createElement('div');imgDiv.innerHTML=imgSrc.innerHTML;expDiv.appendChild(imgDiv);}}\n      card.appendChild(expDiv);cont.appendChild(card);\n    }\n  }\n\n  function pick(qid,oi){\n    var q,opts,i,el,correct;\n    if(answers[qid]!==undefined||done)return;\n    q=null;for(i=0;i<QS.length;i++){if(QS[i].id===qid){q=QS[i];break;}}\n    if(!q)return;\n    opts=shuffled[qid];correct=(opts[oi]===q.correct);\n    answers[qid]=correct?'c':'w';answered++;\n    for(i=0;i<opts.length;i++){\n      el=byId(NS+'-o'+qid+'-'+i);\n      if(opts[i]===q.correct){el.className='mr-opt correct locked';}\n      else if(i===oi){el.className='mr-opt wrong locked';}\n      else{el.className='mr-opt dimmed locked';}\n    }\n    byId(NS+'-exp'+qid).style.display='block';\n    byId(NS+'-pip'+qid).className='mr-pip '+(correct?'correct':'wrong');\n    if(qid>1){var pl=gid('pl'+qid);if(pl)pl.className='mr-pip-line done';}\n  }\n\n  function showScore(){\n    var c,w,s,net,pct,disp,verdicts,vi,sc;\n    if(done)return;done=true;\n    c=countVal('c');w=countVal('w');s=TOTAL-answered;\n    net=(c*4)-w;pct=Math.max(0,Math.round((net\/MAX)*100));disp=Math.min(100,Math.max(0,pct));\n    gid('ring').style.background='conic-gradient(#2A5470 '+disp+'%, #D8E6EE 0%)';\n    gid('pct').textContent=pct+'%';\n    gid('net').textContent='Net Score: '+net+' \/ '+MAX;\n    verdicts=[[5,'Perfect round. 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