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RTA & inherited tubular disorders flashcards

18 high-yield MRCP Part 1 cards from the Intellect Medicos Nephrology course. Try to answer each question before you open it — then study them properly with spaced repetition, which brings each card back just before you would forget it.

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1. What is the defect in type 1 (distal) renal tubular acidosis, and what is the urine pH?

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   Impaired H+ secretion by α-intercalated cells of the collecting duct; urine pH stays above 5.5 despite acidosis.

   Why? It causes a normal anion gap (hyperchloraemic) metabolic acidosis with hypokalaemia.

2. Name three causes of type 1 (distal) RTA.

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   Any three of: Sjögren syndrome, rheumatoid arthritis, SLE, amphotericin B, analgesic nephropathy, medullary sponge kidney.

   Why? Autoimmune damage to the collecting duct or drug toxicity impairs acid secretion.

3. Which renal complication is characteristic of type 1 RTA but not type 2?

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   Nephrocalcinosis and calcium phosphate stones.

   Why? Alkaline urine, hypercalciuria and low urinary citrate favour calcium phosphate precipitation.

4. What is the treatment of type 1 (distal) RTA?

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   Oral alkali (sodium bicarbonate or potassium citrate), with potassium correction and good hydration.

   Why? Correcting acidosis reduces calcium loss from bone and lowers stone formation. Treat the underlying cause.

5. Name three drugs that cause proximal (type 2) RTA or Fanconi syndrome.

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   Any three of: acetazolamide, tenofovir, ifosfamide, outdated (degraded) tetracyclines.

   Why? These impair proximal tubular reabsorption of bicarbonate (and in Fanconi, of glucose, amino acids and phosphate).

6. In type 2 (proximal) RTA, why can the urine pH fall below 5.5?

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   Once serum bicarbonate falls below the lowered reabsorptive threshold, the intact distal nephron can still acidify the urine.

   Why? Treatment needs high-dose oral bicarbonate plus potassium; thiazides can help by causing mild volume contraction.

7. Which type of RTA causes HYPERkalaemia, and what is its commonest cause in adults?

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   Type 4 RTA; diabetic nephropathy (hyporeninaemic hypoaldosteronism).

   Why? Low aldosterone effect reduces potassium and H+ secretion. Drugs (ACEi/ARBs, spironolactone, amiloride, NSAIDs, trimethoprim) and adrenal insufficiency are other causes.

8. Which drug is used to treat type 4 RTA due to aldosterone deficiency?

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   Fludrocortisone.

   Why? It replaces mineralocorticoid action, promoting potassium and acid excretion. Also stop culprit drugs and treat hyperkalaemia.

9. What urinary losses define Fanconi syndrome?

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   Glycosuria (with normal blood glucose), aminoaciduria, phosphaturia and bicarbonaturia (type 2 RTA).

   Why? It is generalised proximal tubular dysfunction. Phosphate loss causes rickets in children and osteomalacia in adults.

10. Cystinosis, the commonest cause of Fanconi syndrome in children, is caused by mutations in the ______ gene and is treated with cysteamine.

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    CTNS

    Why? Cystinosin normally exports cystine from lysosomes. Cystine crystals build up in the proximal tubule and cornea (photophobia). Cysteamine depletes lysosomal cystine.

11. Cystinosis, the commonest cause of Fanconi syndrome in children, is caused by mutations in the CTNS gene and is treated with ______.

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    cysteamine

    Why? Cystinosin normally exports cystine from lysosomes. Cystine crystals build up in the proximal tubule and cornea (photophobia). Cysteamine depletes lysosomal cystine.

12. Cystinuria impairs proximal tubular reabsorption of which four amino acids?

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    Cystine, ornithine, lysine and arginine.

    Why? Only cystine is poorly soluble, so it forms recurrent stones with hexagonal crystals. It is autosomal recessive (SLC3A1/SLC7A9).

13. What is the management of cystinuria?

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    High fluid intake, urinary alkalinisation (potassium citrate), and a chelating agent such as penicillamine if needed.

    Why? Cystine is more soluble in alkaline urine. Penicillamine forms a soluble disulphide with cysteine.

14. Classic homocystinuria is due to deficiency of which enzyme, and what is the first-line treatment?

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    Cystathionine β-synthase; pyridoxine (vitamin B6), with folate and B12.

    Why? Features are a marfanoid habitus, downward (inferonasal) lens dislocation, learning difficulty and thrombosis. About half respond to pyridoxine.

15. Bartter syndrome: which transporter is defective, and what happens to urinary calcium?

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    NKCC2 in the thick ascending limb; urinary calcium is HIGH (hypercalciuria).

    Why? It mimics chronic loop diuretic use: hypokalaemic metabolic alkalosis with normal BP despite high renin and aldosterone. Usually presents in infancy or childhood.

16. Which two biochemical features distinguish Gitelman syndrome from Bartter syndrome?

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    Hypomagnesaemia and hypocalciuria.

    Why? Gitelman is a defect of the thiazide-sensitive NCC in the DCT (SLC12A3), so it mimics a thiazide. It is milder and presents in adolescence or adulthood. Treat with potassium and magnesium replacement.

17. What is the treatment of Liddle syndrome, and why does spironolactone not work?

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    Amiloride or triamterene (ENaC blockers); spironolactone fails because aldosterone is already suppressed.

    Why? Liddle syndrome is an autosomal dominant gain-of-function mutation of ENaC: hypertension, hypokalaemia, metabolic alkalosis with LOW renin and LOW aldosterone.

18. How does excessive liquorice cause hypertension and hypokalaemia?

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    Glycyrrhetinic acid inhibits 11β-hydroxysteroid dehydrogenase type 2, so cortisol activates mineralocorticoid receptors.

    Why? This is apparent mineralocorticoid excess — renin and aldosterone are both low. Carbenoxolone acts the same way.