Consumption of which of the following poison produce bluish discoloration of stomach during postmortem examination.
Sodium amytal
Soneryl
Oxalic acid
Arsenic
Ref. Textbook of forensic medicine. Krishnan Vij. Page. 586
 
Barbiturate poisoning

They are rapidly absorbed from the gastrointestinal tract including the rectum.
They are concentrated in the liver for a short time and then distributed into the body tissues and fluids
Long-acting, 4–7gm; intermediate-acting, 2–3gm; short- and ultrashort-acting, 1.5–2 gm.
Plasma levels of 3.5 mg/dl for short- acting and 10 mg/dl for long-acting barbiturates are indicative of serious toxicity.
Cyanosis is usually present. Postmortem staining may be promi- nent. In a few cases, there may be skin blisters, the so-called barbiturate blisters.
They are commonly found at the sites where pressure has been exerted between the skin surfaces, such as buttocks, backs of thighs, calves and forearms.
Inner surface of stomach has bluish and whitish barbiturate particles