Ans. B. CO. (Ref. Bailey & Love 26th ed. Pg. 386; Summary box 28.3)Metabolic poisoning in burns:# There are many poisonous gases that can be given off in a fire, the most common being carbon monoxide, a product of incomplete combustion that is often produced by fires in enclosed spaces. This is the usual cause of a person being found with altered consciousness at the scene of a fire.# Carbon monoxide binds to haemoglobin with an affinity 240 times greater than that of oxygen and therefore blocks the transport of oxygen. Levels of carboxyhaemoglobin in the bloodstream can be measured. Concentrations above 10% are dangerous and need treatment with pure oxygen for more than 24 hours. Death occurs with concentrations around 60%.# Another metabolic toxin produced in house fires is hydrogen cyanide, which causes a metabolic acidosis by interfering with mitochondrial respiration.Additional educational points:The shock reaction after burns# Bums produce an inflammatory reaction# This leads to vastly increased vascular permeability# Water, solutes and proteins move from the intra- to the extravascular space# The volume of fluid lost is directly proportional to the area of the bum# Above 15% of surface area, the loss of fluid produces shockASPHYXIANTS# Include CO, CO2, Hydrogen sulphide and some war gases.- CO reduces the oxygen carrying capacity of the blood;- Carbon dioxide produces oxygen lack in the tissues;- Hydrogen sulphide paralyses the respiratory centre.Cause of death in burns:# Primary neurogenic shock - instantaneously from fear or pain or within 24-48 hrsfrom severe pains or from injury to vital organ from burning, or from oligemic shock.# If a victim survives this stage, this stage merges rapidly into the stage of secondary shock due to depletion of blood (serum) volume.# Apart from actual burning, death may occur from asphyxia (suffocation) due to inhalation of smoke containing carbon dioxide, carbon monoxide, and other products of combution, which may be poisonous (in recent yrs cyanide intoxication has been recognized as important, especially where plastics and paints are burning), or accidental injuries from falling structures whle trying to escape from abuming house, or from fat embolism.