**Question:** Characteristic of type-II respiratory failure is:September 2010

**Core Concept:**
Type II respiratory failure, also known as acute respiratory acidosis, is a form of respiratory failure where the primary defect lies in the ventilatory control system. It occurs due to inadequate respiratory effort or an impaired chemosensitivity, leading to insufficient CO2 removal despite adequate lung compliance and ventilation-perfusion matching.

**Why the Correct Answer is Right:**
The correct answer is type C (acidosis). Acidosis is a condition where acidic waste products accumulate in the blood due to inadequate respiratory compensation. In type II respiratory failure, the primary defect is in the ventilatory control system, leading to an increased PCO2 (arterial partial pressure of carbon dioxide) and decreased pH (acidosis).

**Why Each Wrong Option is Incorrect:**
A. Type I respiratory failure (alveolar hypoventilation) is characterized by low PaCO2 (arterial partial pressure of carbon dioxide) and normal or increased pH.
B. Type III respiratory failure (hypoventilation) is characterized by normal or increased PaCO2 and decreased pH.
D. Type IV respiratory failure (pulmonary fibrosis) is a lung disease characterized by scarring and reduced lung function, unrelated to respiratory acidosis.

**Clinical Pearl / High-Yield Fact:**
In type II respiratory failure, the acidosis is primarily due to the accumulation of acidic waste products in the blood (increased anion gap acidosis). This type of acidosis is crucial to understand as it helps in differentiating it from other acid-base imbalances, such as renal acidosis or alkalosis, which have different underlying causes and treatment strategies.

**Correct Answer:**
Type C (acidosis)

**Explanation:**
Type II respiratory failure results in increased anion gap acidosis, as it is characterized by the accumulation of acidic waste products, such as lactate and ketone bodies, in the blood due to impaired respiratory compensation. This acidosis is commonly seen in conditions like obesity hypoventilation syndrome, neuromuscular diseases, and pulmonary diseases like chronic obstructive pulmonary disease (COPD) or restrictive lung diseases like restrictive lung disease due to skeletal muscle weakness.