What is the most likely diagnosis for a 21‑year‑old normotensive male with acute flaccid lower‑motor‑neuron paralysis, normal‑anion‑gap metabolic acidosis (pH 7.29, bicarbonate ≈ 12 mmol/L), hypokalaemia (K⁺ ≈ 2.6 mmol/L), low BUN/creatinine ratio, and ECG T‑wave flattening?

Diagnosis: Distal (Type 1) Renal Tubular Acidosis

The most likely diagnosis is distal (type 1) renal tubular acidosis (RTA), which classically presents with acute flaccid paralysis from severe hypokalaemia, normal‑anion‑gap metabolic acidosis, and alkaline urine in a young patient with recurrent episodes. 1, 2

Clinical Reasoning and Diagnostic Features

Key Laboratory Findings Supporting Distal RTA:

• Normal‑anion‑gap metabolic acidosis (pH 7.29, bicarbonate 11.6 mmol/L) with hyperchloraemia is the hallmark of all RTA subtypes, distinguishing them from high‑anion‑gap causes such as ketoacidosis or toxic ingestions 1, 2, 3

• Severe hypokalaemia (K⁺ 2.6 mmol/L) causing flaccid lower‑motor‑neuron paralysis is characteristic of distal RTA, where the inability to excrete hydrogen ions leads to persistent urinary potassium wasting 1, 2

• ECG T‑wave flattening directly reflects the severe hypokalaemia and indicates risk for life‑threatening cardiac arrhythmias 1, 4

• Low BUN/creatinine ratio (0.8) argues against stage V chronic kidney disease, which would typically show markedly elevated BUN and creatinine with a ratio >20:1 5, 6

• Recurrent episodes ("similar history few months back") are typical of distal RTA, particularly when associated with autoimmune conditions such as Sjögren syndrome that may present initially with episodic paralysis 4, 7

Differential Diagnosis Exclusion

Why NOT Alcoholic Ketoacidosis:

• Alcoholic ketoacidosis produces a high‑anion‑gap metabolic acidosis with positive serum/urine ketones and typically low‑to‑normal glucose 8
• The normal anion gap (calculated as Na⁺ − [HCO₃⁻ + Cl⁻] ≈ 146 − [11.6 + Cl⁻]) in this patient excludes ketoacidosis 8, 3
• No clinical history of alcohol use is mentioned 8

Why NOT Ethylene Glycol Intoxication:

• Ethylene glycol poisoning causes a high‑anion‑gap metabolic acidosis with elevated osmolal gap and typically presents acutely with altered mental status, not recurrent episodes 8
• The normal anion gap excludes toxic ingestions 8, 3
• The recurrent nature of episodes over months is inconsistent with acute poisoning 8

Why NOT Stage V CKD:

• Stage V CKD (GFR <15 mL/min/1.73 m²) would show **markedly elevated BUN and creatinine** with a BUN/creatinine ratio typically >20:1, not 0.8 5, 6
• While CKD can cause metabolic acidosis, it typically presents with a high or moderately elevated anion gap due to retained uremic anions, not a normal anion gap 6
• A 21‑year‑old with normotension and recurrent acute episodes is atypical for advanced CKD, which usually develops over years with hypertension and progressive decline 6

Why NOT Proximal (Type 2) RTA:

• Proximal RTA typically presents with less severe hypokalaemia and milder acidosis (bicarbonate usually 12–20 mmol/L) because bicarbonate wasting is self‑limited once plasma levels fall below the reabsorptive threshold 1, 2
• Proximal RTA is often part of Fanconi syndrome with additional features such as hypophosphatemia, hypouricemia, glycosuria, and aminoaciduria 5, 7
• The severe hypokalaemia (2.6 mmol/L) with profound acidosis (bicarbonate 11.6 mmol/L) and recurrent paralysis are more consistent with distal RTA 1, 2, 4

Diagnostic Confirmation Algorithm

To definitively confirm distal RTA, perform the following tests:

1. Urine pH measurement during acidemia: Urine pH persistently >5.5 despite systemic acidosis (pH 7.29, bicarbonate 11.6 mmol/L) confirms distal RTA, as the distal tubule cannot acidify urine appropriately 1, 2, 3

2. Urine anion gap calculation: A positive urine anion gap (Na⁺ + K⁺ − Cl⁻ > 0) indicates impaired ammonium excretion, characteristic of distal RTA, whereas a negative gap suggests gastrointestinal bicarbonate loss 3, 4, 7

3. Serum potassium and bicarbonate monitoring: Document persistent hypokalaemia with renal potassium wasting (transtubular potassium gradient >4 despite hypokalaemia) 1, 2

4. Screen for underlying autoimmune disease: Check anti‑Ro (SS‑A), anti‑La (SS‑B), ANA, and Schirmer test to identify Sjögren syndrome, a common cause of distal RTA in young adults 4, 7

Immediate Management Priorities

Acute Treatment:

• Intravenous potassium replacement is urgent to prevent life‑threatening arrhythmias and respiratory failure; administer potassium chloride 10–20 mEq/h with continuous cardiac monitoring until K⁺ >3.0 mmol/L 1, 4

• Oral alkali therapy with potassium citrate (1–2 mEq/kg/day divided into 2–3 doses) addresses both the acidosis and hypokalaemia, providing bicarbonate equivalents while repleting potassium 1, 2

• Avoid sodium bicarbonate alone in the acute setting, as it can worsen hypokalaemia by driving potassium intracellularly without providing potassium replacement 1

Long‑Term Management:

• Maintain serum bicarbonate ≥22 mmol/L with chronic potassium citrate or sodium bicarbonate supplementation to prevent bone demineralization, nephrocalcinosis, and renal calculi 1, 2

• Monitor serum potassium monthly and adjust alkali dosing to maintain K⁺ >3.5 mmol/L 1

• Investigate and treat underlying autoimmune disease if Sjögren syndrome or other systemic disorder is identified 4, 7

Critical Clinical Pitfalls to Avoid

• Do not assume proximal RTA based solely on normal‑anion‑gap acidosis; the severity of hypokalaemia and acidosis with recurrent paralysis strongly favors distal RTA 1, 2

• Do not overlook autoimmune screening in young patients with distal RTA, as Sjögren syndrome may present initially with episodic paralysis before sicca symptoms become apparent 4, 7

• Do not delay potassium replacement while awaiting confirmatory urine studies; severe hypokalaemia with paralysis and ECG changes requires immediate treatment to prevent cardiac arrest 1, 4

• Do not confuse the low BUN/creatinine ratio with advanced CKD; this ratio reflects the patient's young age and normal renal function, not end‑stage disease 5, 6