Answer is A (Central cyanosis becomes evident when reduced Hb < 5g/dl): Cyanosis (both central and peripheral) become evident when the amount of reduced Hb exceeds 5g/dl (becomes >5gm/dI) and not when reduced Hb is less than 5g/d1. Cyanosis becomes evident when Reduced Hb exceeds 'Cyanosis becomes evident when reduced Hb exceeds `Cyanosis becomes evident when reduced Hh exceeds Pulmonary diseases such as chronic Asthma, Alveolar hypoventilation and Hemoglobin disorders like methemoglobinemia all cause central cyanosis Central cyanosis Central cyanosis is caused by in adequate oxygenation of systemic aerial blood due to decreased aerial oxygen saturation or Hemoglobin abnormalities It results from cardiac shunting, pulmonary diseases or hematological disorders (Hb abnormalities) Central cyanosis is charachterized by cyanosis affective both mucous membranes and skin Causes Decreased aerial oxygen saturation Decreased atmospheric pressure - high altitude Impaired pulmonary function - Alveolar hyoventilation - Uneven relationships between pulmonary ventilation and perfusion (perfusion of hypoventilated alveoli) - Impaired oxygen diffusion Anatomic shunts - Ceain types of congenital hea disease - Pulmonary aeriovenous fistulas - Multiple small intrapulmonary shunts Hemoglobin with low affinity for oxygen Hemoglobin abnormalities *Methemoglobinemia - hereditary, acquired Sulfhemoglobinema - acquired Carboxyhemoglobinemia (not true cyanosis) Peripheral cyanosis Peripheral cyanosis is caused by slowing of blood flow and increased extraction of oxygen from normally saturated blood It results from vasoconstriction or decreased peripheral blood flow, reduced cardiac output or vascular occlusion Peripheral cyanosis is characterized by cyanosis of skin alone and sparing of mucous membranes Causes Vascular occlusion - Aerial obstruction - Venous obstruction Reduced cardiac output Cold exposure Redistribution of blood flow from extremities