Metabolic acidosis is a condition characterized by low blood pH due to excess acid or bicarbonate loss, and it directly lowers plasma bicarbonate levels as the primary hallmark.

Correct Answer

(A) bicarbonate
Metabolic acidosis features a primary drop in plasma bicarbonate (HCO₃⁻), typically below 22 mmol/L, alongside low pH (<7.35), as acids overwhelm buffering systems.

Option Analysis

• (A) Bicarbonate: Correct. Bicarbonate acts as the key buffer; its plasma level decreases directly in metabolic acidosis from acid addition (e.g., lactic acid) or loss (e.g., diarrhea), confirmed across clinical definitions.

• (B) Lactate: Incorrect. Lactate often rises in metabolic acidosis, especially type A lactic acidosis from hypoxia, where levels exceed 4-5 mmol/L, contributing to the acid load rather than decreasing.

• (C) Oxygen: Incorrect. Plasma oxygen (as PaO₂) shows no consistent decrease; acidosis shifts the oxyhemoglobin curve rightward initially to aid unloading, but oxygen levels relate more to respiratory or perfusion issues.

• (D) Urea: Incorrect. Urea levels may rise in associated renal failure (uremic acidosis), but correction of acidosis can lower elevated urea; it does not characteristically decrease.

Introduction to Metabolic Acidosis Decreased Plasma Levels

Metabolic acidosis decreased plasma bicarbonate levels defines this acid-base disorder, where blood pH drops below 7.35 due to bicarbonate (HCO₃⁻) falling under 22 mmol/L from acid gain or base loss. Critical for CSIR NET Life Sciences, this MCQ tests core biochemistry knowledge on plasma changes.

Causes of Decreased Plasma Bicarbonate

• High anion gap types: Lactic acidosis (hypoxia), ketoacidosis (diabetes), toxins raise acids, consuming bicarbonate.

• Normal anion gap: Diarrhea (HCO₃⁻ loss), renal tubular acidosis impair regeneration.
  Renal failure adds uremic acids, further dropping bicarbonate.

Why Not Lactate, Oxygen, or Urea?

Lactate increases as a cause in many cases, not decreases. Oxygen delivery adjusts via curve shifts, unchanged in plasma. Urea elevates in renal-linked acidosis.

Symptoms and Diagnosis

Symptoms include Kussmaul breathing, fatigue, confusion; diagnose via ABG (low pH, HCO₃⁻), anion gap. Compensation lowers PaCO₂ by 1-1.3 mmHg per 1 mEq/L HCO₃⁻ drop.

CSIR NET Exam Relevance

This MCQ reinforces acid-base physiology for Part B/C, linking to enzyme kinetics, renal function. Focus on bicarbonate as the defining decreased plasma marker.