The following are true for renal tubular acidosis (RTA) except:
Answer D
Renal tubular acidosis (RTA) is a family of diseases characterised by failure of tubular H+ secretion and urinary acidification. Type 1 RTA is the failure of H+ secretion in the distal nephron tubules and is characterised by hyperchloraemic metabolic acidosis (normal anion gap), a high urinary pH (>5.5, alkaline urine) and low serum bicarbonate. The disease also has low sodium levels, there is a female predominance and due to the low citrate they are predisposed to calcium stones and in particular calcium phosphate stones. The condition is treated with potassium citrate solution. In Type 2 RTA, there is a failure of bicarbonate reabsorption (loss) in the proximal tubule of the nephron with similar characteristics to Type 1 RTA except citrate levels are normal therefore no stones form. The ammonium chloride acidification test (100 mg/kg) is one of the tests used to diagnose RTA with a urinary pH of >5.5 indicative of a failure of urinary acidification, and supportive of the diagnosis of dRTA.
Treatment of distal RTA include correction of the metabolic acidosis, with potassium citrate being one of the treatment options.
The following are true of hypercalciuria and its relation to stone formation, except:
Answer C
Hypercalciuria is defined as >4 mg/kg/24 hours or >7 mmol (men) or >6 mmol (women). Hypercalciuria can classified into idiopathic (50%), absorptive (from gut), renal leak or resorptive (from bone). In absorptive hypercalciuria, excessive calcium is absorbed from the gut leading to increased renal filtration and reduced renal reabsorption due to low parathyroid hormone and associated raised urinary phosphate (fasting urinary calcium is normal). Impaired tubular reabsorption of calcium (renal leak) occurs in 5%–10% of calcium stone formers and is characterised by fasting hypercalciuria with secondary hyperparathyroidism (raised PTH) but without hypercalcaemia. Resorptive hypercalciuria is almost always due to primary hyperparathyroidism which accounts for 3%–5% of all cases of hypercalciuria. The increased PTH levels leads to release of calcium from the bones as well as increasing calcium and vitamin D absorption from the bone and reducing calcium renal excretion from the distal tubule resulting in hypercalciuria (Table below). Excess salt intake can result in hypercalciuria, as sodium and calcium are co-transported in the kidney. Hypercalciuria is most commonly associated with calcium oxalate dihydrate stones.
Summary of the discriminating features of the different causes of hypercalciuria:
Which of the following is the most important factor in uric acid stone formation?
Answer A
The most important factor in uric acid stone formation is low urinary pH as most patients have normal urinary uric acid levels. Uric acid solubility is significantly reduced when urinary pH is <5.5. Other important factors include low urine volume and a hyperuricosuria. Low urinary volume can more commonly occur in patients with chronic diarrhoeal conditions, ileostomies, excessive sweating or poor oral intake. Hyperuricosuria occurs in addition to hyperuricaemia in patients with primary gout, myeloproliferative conditions and Lesch-Nyhan syndrome. Hyperuricosuria occurs in patients without raised serum urate levels due to some medications (thiazides or salicylates) and excessive intake of dietary meats.
The following antibiotics are safe in one or all trimesters of pregnancy, except:
Answer E
Penicillin, cephalosporin and macrolide antibiotics are considered safe to use in pregnancy. Nitrofurantoin is also safe for most of pregnancy but should be avoided towards term due to an increased risk of neonatal haemolysis. Trimethoprim is a folate antagonist and should be avoided, especially in the first trimester during organogenesis. There is a risk of auditory or vestibular nerve damage with gentamicin and therefore should not be used in pregnancy. There is limited information on tazocin or carbapenems with manufactures advising to use only if the potential benefit outweighs the risk in more severe infections.
The most common type of urinary stone is:
The most common types of urinary stones are composed of calcium oxalate (60%–70%) followed by calcium phosphate (10%–20%), infection stones (10%–15%) and then uric acid stones (5%–10%). Cystine stones occur in <1% of cases.
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