Which of the following is the most common primary cause of respiratory alkalosis?

• A. Increased production of ketoacids in uncontrolled diabetes mellitus
• B. Loss of gastric acid from prolonged vomiting causing increased plasma ${HCO}^{-}$
• C. Hypoventilation leading to increased arterial $P_{aCO2}$ (hypercapnia)
• D. Hyperventilation leading to decreased arterial $P_{aCO2}$ (hypocapnia)

Answer: (D)

Verified step by step guidance

1. Step 1: Understand the definition of respiratory alkalosis. Respiratory alkalosis occurs when there is a decrease in arterial carbon dioxide pressure (PaCO2), leading to an increase in blood pH (alkalemia). This is typically caused by excessive loss of CO2 through the lungs.
2. Step 2: Review the causes of respiratory alkalosis. The primary cause is hyperventilation, which leads to excessive exhalation of CO2, reducing PaCO2 and causing the blood to become more alkaline.
3. Step 3: Analyze the other options given: increased production of ketoacids in uncontrolled diabetes mellitus causes metabolic acidosis, not respiratory alkalosis; loss of gastric acid from vomiting leads to metabolic alkalosis due to loss of H+ ions; hypoventilation causes respiratory acidosis due to increased PaCO2 (hypercapnia).
4. Step 4: Recognize that hyperventilation decreases arterial PaCO2 (hypocapnia), which is the hallmark of respiratory alkalosis. This is the most common primary cause of respiratory alkalosis.
5. Step 5: Conclude that the correct answer is hyperventilation leading to decreased arterial PaCO2 (hypocapnia), as it directly causes respiratory alkalosis by lowering CO2 levels and increasing blood pH.