Renal Physiology and Clinical Approach — Advanced

Muddy brown granular casts and renal tubular epithelial cell casts on urine microscopy are highly specific for acute tubular injury. Diuretics can raise urine sodium and FENa, making those indices unreliable for distinguishing pre-renal physiology from ATN. FEUrea may be useful when diuretics confound FENa, but the question asks for the most specific indicator of ATN. Urine osmolality and spot urine sodium are also affected by diuretic therapy.

Correct answer: Intravenous calcium (option E). The immediate priority in hyperkalaemia with ECG changes is to stabilise the myocardium with IV calcium (calcium gluconate or calcium chloride) to reduce the risk of malignant arrhythmia. This does not lower serum potassium but protects the heart while definitive potassium-lowering therapies are given. Insulin with glucose (option A) and nebulised salbutamol (option B) are rapid potassium-shifting therapies and should follow calcium for membrane stabilisation. IV furosemide (option C) can promote renal potassium excretion if effective urine output will occur but is not the initial life-saving intervention. Oral ion-exchange resins (option D) act slowly and are not appropriate as first-line acute therapy for ECG changes.

Correct answer: Cystatin C–based estimated GFR (option A). Cystatin C is less dependent on muscle mass than creatinine and gives a more accurate estimate of GFR in patients with low muscle mass/sarcopenia. Measured GFR with exogenous filtration markers (eg, iohexol) is the gold standard but is not listed here. A 24-hour urine creatinine clearance (option B) is error-prone (collection inaccuracies) and overestimates GFR due to tubular secretion; it is also still creatinine-based and affected by low muscle mass. Cockcroft–Gault (option C) is an estimate of creatinine clearance using serum creatinine and body weight and is unreliable in extremes of muscle mass. Serum creatinine alone (option D) is inadequate in sarcopenia. Urinary ACR (option E) assesses albuminuria rather than GFR.

Correct answer: High-anion-gap metabolic acidosis without an additional metabolic disorder (option D). Anion gap = Na − (Cl + HCO3) = 140 − (100 + 12) = 28 mmol/L (elevated; normal ~12). Delta AG = 28 − 12 = 16; delta HCO3 = 24 − 12 = 12; the delta/delta ratio ≈ 1.33, which falls within the 1–2 range consistent with a pure high-anion-gap metabolic acidosis (no concurrent normal anion gap acidosis or metabolic alkalosis). Option A is incorrect because the anion gap is elevated (so it is not a normal AG acidosis). Option B (mixed high AG + normal AG) would be suggested by a delta/delta <1. Option C and E are incorrect because the primary disorder is a metabolic acidosis (not primary respiratory or alkalosis).

Correct answer: Membranous nephropathy with severe hypoalbuminaemia (option B). Membranous nephropathy is associated with the highest risk of venous thromboembolism among nephrotic syndromes, particularly when serum albumin is markedly low; such patients are most commonly considered for individualized prophylactic anticoagulation after weighing bleeding risk. Diabetic nephropathy (option A) typically has lower thrombotic risk attributable to nephrosis per se. Minimal change disease (option C) can be associated with nephrotic-range proteinuria and thrombotic risk if albumin is severely low, but membranous has the highest incidence. FSGS (option D) has increased risk but less than membranous on average. Immune complex GN with low cholesterol (option E) is not the typical high-thrombosis profile.

Correct answer: Stop nephrotoxins and optimise haemodynamics (option E). Muddy brown granular casts indicate acute tubular necrosis (ATN) due to ischaemic injury. Management is supportive: restore and maintain adequate renal perfusion, correct volume status, avoid or stop nephrotoxins, and treat the underlying cause. Renal biopsy (option A) is rarely required in typical ATN and is not first-line. High-dose loop diuretics (option B) do not improve renal recovery and are not recommended routinely to convert oliguria into non-oliguria. IV albumin (option C) may be used in specific circumstances (eg, hypoalbuminaemia with intravascular depletion) but is not universally indicated. Immediate initiation of chronic haemodialysis (option D) is unnecessary unless the patient has urgent indications for renal replacement therapy (refractory hyperkalaemia, severe acidaemia, volume overload or uraemia).

Correct answer: Add an oral potassium-binding therapy (option C). Contemporary guidance supports using measures to control recurrent hyperkalaemia (including oral potassium binders) to enable continuation of renin–angiotensin system (RAS) inhibitors when these agents provide clear renal and cardiovascular benefit. Permanently stopping the ACE inhibitor (option A) may worsen long-term kidney and cardiovascular outcomes and should be a last resort after considering mitigation strategies. Diet alone (option B) may help but is often insufficient for recurrent hyperkalaemia. Thiazide diuretics (option D) have limited efficacy at low eGFR and may be ineffective; loop diuretics are favoured if diuretics are required. Doing nothing (option E) is inappropriate because recurrent hyperkalaemia risks arrhythmia and may require prompt management.

Correct answer: Start urgent renal replacement therapy for refractory severe metabolic acidaemia (option A). Severe refractory acidemia (eg, pH ≤7.1 that does not respond to supportive measures) is an established indication for urgent RRT because it can cause haemodynamic instability and reduced myocardial contractility. Repeated bicarbonate boluses (option B) are of limited benefit and can cause volume overload and sodium load; they may be used transiently but are not a substitute for RRT when acidaemia is refractory. Non-invasive ventilation (option C) is not a primary therapy for metabolic acidosis. IV albumin (option D) is not indicated to correct acid–base disturbance in this context. Arranging outpatient dialysis (option E) is inappropriate for an acute, life-threatening metabolic derangement.

Correct answer: Distal (type 1) renal tubular acidosis (option B). In a normal anion gap metabolic acidosis, a urine pH >5.5 suggests impaired distal acidification consistent with distal RTA. A positive urine anion gap indicates low urinary ammonium excretion (low NH4+), supporting a renal cause for reduced acid excretion rather than gastrointestinal bicarbonate loss. Proximal RTA (option A) typically shows variable urine pH (can be <5.5 once plasma bicarbonate stabilises) and is associated with marked bicarbonaturia and other proximal tubular defects. Type 4 RTA (option C) is associated with hyperkalaemia and often a low urine pH may be seen; UAG findings vary. Gastrointestinal bicarbonate loss (option D) typically results in a negative UAG due to appropriate renal NH4+ excretion. Respiratory acidosis with renal compensation (option E) is not relevant to these urinary indices.

Correct answer: Replace urine losses with isotonic crystalloid guided by measured urine output and monitor electrolytes closely (option D). Post-obstructive diuresis can be physiologically appropriate and large, causing hypovolaemia and electrolyte losses (sodium, potassium). Management is to match losses with isotonic fluid (eg, 0.9% saline) guided by urine output, correct electrolyte deficits (eg, potassium), and monitor vitals and biochemistry frequently. Fluid restriction (option A) is inappropriate in the presence of ongoing large-volume losses and falling blood pressure. Hypotonic saline (option B) risks worsening hyponatraemia if sodium loss predominates; isotonic fluid is preferred for volume repletion. Oral potassium alone (option C) is insufficient when intravascular volume needs replacement and the patient may require IV supplementation. Immediate renal replacement therapy (option E) is not indicated to “stop” diuresis and would not address hypovolaemia.