**Question:** A patient presents with respiratory distress and shows pH = 7.2, PCO2 = 50 mmHg, PO2 = 85 mmHg, and HCO3 = 20 mEq/L. The cause is

A. Acidosis
B. Hypercapnia
C. Hypoxemia
D. Hyperventilation

**Correct Answer:** D. Hyperventilation

**Core Concept:**
Understanding respiratory acid-base balance and its disturbances is crucial in identifying and managing respiratory disorders. Respiratory acid-base disorders are classified into two main types: respiratory acidosis and respiratory alkalosis. Respiratory acidosis occurs when carbon dioxide (CO2) levels increase in the blood, while respiratory alkalosis occurs when CO2 levels decrease. These changes are reflected in the pH, PCO2, PO2, and HCO3 levels.

**Why the Correct Answer is Right:**
In this case, the patient exhibits the following parameters: pH = 7.2, PCO2 = 50 mmHg, PO2 = 85 mmHg, and HCO3 = 20 mEq/L. These values indicate respiratory acidosis. A low pH (7.2) signifies an acidic environment, while elevated PCO2 (50 mmHg) and decreased HCO3 (20 mEq/L) confirm the presence of increased CO2 levels. The normal PO2 (85 mmHg) suggests respiratory rather than hypoxic acidosis.

**Why Each Wrong Option is Incorrect:**
A. Acidosis: While the patient has an acidic pH, the other parameters do not align with typical acidosis (increased PCO2 and decreased HCO3).
B. Hypercapnia: Although elevated PCO2 is present, this option focuses on CO2 levels, which are not the primary cause of respiratory acidosis.
C. Hypoxemia: The normal PO2 indicates that hypoxic acidosis is not the cause.
D. Hypoventilation: This option suggests reduced CO2 levels, which contradicts the observed elevated PCO2.

**Clinical Pearl:**
In respiratory acidosis, hyperventilation (option D) is a compensatory mechanism to decrease CO2 levels and restore respiratory equilibrium. However, this case demonstrates increased CO2 levels, not reduced ones. Therefore, the correct answer is respiratory acidosis, which results from excessive ventilation (hyperventilation) and decreased CO2 levels, rather than hypoventilation. This mechanism helps maintain acid-base balance and oxygenation, but in this situation, it causes the elevated CO2 levels and acidemia.