The FALSE Statement is E: Acute Renal Failure DOES Cause Metabolic Acidosis
Statement E is NOT true—acute renal failure is a well-established cause of metabolic acidosis, not an exception to it.
Analysis of Each Statement
Statement A: TRUE
Metabolic acidosis is indeed characterized by low HCO3- levels and acidemia (pH <7.35). This is the fundamental definition supported by guideline evidence 1. In all forms of metabolic acidosis, there is a low blood bicarbonate level, either due to loss of bicarbonate or due to buffering of excess acid by bicarbonate 2, 3.
Statement B: TRUE
Metabolic acidosis is correctly categorized into:
- High anion gap metabolic acidosis (anion gap >12 mEq/L)
- Normal anion gap metabolic acidosis (hyperchloremic acidosis)
This classification is the standard diagnostic approach and helps identify the underlying etiology 2, 4, 5.
Statement C: TRUE
High anion gap metabolic acidosis is caused by accumulation of endogenous acids, including:
- Ketoacidosis (diabetic, alcoholic, starvation)
- Lactic acidosis (tissue hypoxia, sepsis, low cardiac output states)
- Renal failure
- Toxic ingestions
The guideline explicitly states: "A common cause of metabolic acidosis is lactic acidosis caused by tissue hypoxia" 1. Ketoacidosis from diabetic or abnormal metabolic conditions is also clearly documented 1, 4.
Statement D: TRUE
Normal anion gap (hyperchloremic) metabolic acidosis is primarily caused by:
- Gastrointestinal HCO3- losses (chronic diarrhea, fistulas)
- Renal tubular acidosis
- Early renal failure
The guideline states: "it may result from direct loss of bicarbonate from the kidney or gut (e.g., during chronic diarrhoea)" 1. This is the most common extrarenal cause 4, 6.
Statement E: FALSE—This is the Answer
Acute renal failure DOES cause metabolic acidosis. The guideline explicitly states: "This can be caused by failure to excrete acid produced by the body's normal metabolic processes (e.g., during renal failure)" 1.
Why Renal Failure Causes Metabolic Acidosis
The kidney maintains acid-base homeostasis through:
- Elimination of protons (hydrogen ions)
- Reabsorption/generation of bicarbonate
When renal function fails, these mechanisms are impaired, leading to:
- Accumulation of nonvolatile acids from normal metabolism 5
- Impaired ammonium excretion 7
- Reduced bicarbonate regeneration 6
Initially, renal failure causes high anion gap metabolic acidosis due to accumulation of unmeasured anions (sulfate, phosphate, organic acids) 4, 8. With severe GFR reduction, the anion gap increases further 6.
Clinical Pitfall
Do not confuse the type of acidosis with whether renal failure causes acidosis:
- Early/mild renal failure → normal anion gap acidosis
- Advanced renal failure → high anion gap acidosis
- Both are still metabolic acidosis caused by renal failure
The statement that "acute renal failure does not cause metabolic acidosis" contradicts fundamental renal physiology and multiple guideline sources 1, 4, 8, 7.