**Question:** Deep blue colour of hypostasis is seen in death due to poisoning by:

A. Cyanide
B. Arsenic
C. Antimony
D. Mercury

**Core Concept:** Hypostasis is a condition where the body's blood is pooled in the lower extremities, causing a bluish discoloration due to a decrease in oxygen-rich blood flow to the skin. This is a common finding in the post-mortem examination and can indicate the cause of death as well as the time of death.

**Why the Correct Answer is Right:** The deep blue color of hypostasis is primarily due to decreased oxygen-rich blood flow to the skin, leading to reduced oxygenation of the skin cells. In the case of poisoning by cyanide, arsenic, or mercury, these substances can cause direct toxicity to the red blood cells (erythrocytes) or affect the respiratory system, leading to decreased oxygenation and subsequently, the bluish discoloration of the skin seen in hypostasis.

**Why Each Wrong Option is Incorrect:**

A. Antimony poisoning rarely causes direct toxicity to erythrocytes, making it less likely to result in hypostasis with its typical blue-black discoloration.
B. Arsenic poisoning primarily affects the gastrointestinal tract, causing gastrointestinal bleeding, rather than directly affecting the red blood cells or causing hypostasis.
C. Antimony poisoning rarely causes direct toxicity to erythrocytes, making it less likely to result in hypostasis with its typical blue-black discoloration.
D. Mercury poisoning can affect the red blood cells directly, but the primary cause of hypostasis in this scenario would be the mercury's interference with the respiratory system, leading to respiratory failure and decreased oxygenation.

**Clinical Pearl:** Hypostasis can be a useful clue in identifying the cause of death, particularly when combined with other findings. It is essential to consider the clinical context, time of death, and the potential toxic substances involved when interpreting the presence of hypostasis.

**Correct Answer:** D. Mercury

**Explanation:** Mercury poisoning can affect the red blood cells directly, leading to hemolysis and hemoglobinuria. This causes a direct toxic effect on the red blood cells, resulting in hemoglobinuria and subsequent hypostasis. Additionally, mercury can interfere with the respiratory system, causing respiratory failure and decreased oxygenation, contributing to the hypostasis and its blue-colored appearance. Mercury poisoning is a rare cause of hypostasis, but it is essential to consider this possibility when evaluating the presence of hypostasis, especially when combined with other findings such as hemoglobinuria.