**Question:** Cherry red colour of blood is seen in poisoning with

A. Carbon monoxide
B. Cyanide
C. Aspirin
D. Nitrite

**Core Concept:**
The cherry red coloration of blood is due to the presence of highly concentrated oxygen-carrying hemoglobin in the capillaries, which appears red under direct light examination due to light absorption by deoxygenated hemoglobin. The cherry red spot is a useful clinical sign in identifying various conditions affecting the central retina, such as central retinal artery occlusion (CRAO), which is a life-threatening emergency.

**Why the Correct Answer is Right:**
Carbon monoxide (CO) is a toxic gas produced during incomplete combustion of carbon-containing fuels. It binds to hemoglobin (Hb) with high affinity and competes with oxygen (O2) binding, leading to a decrease in oxygen delivery to tissues and organs. In severe poisoning, the cherry red coloration of the retina is due to the accumulation of CO-Hb, which does not absorb red light, unlike oxyhemoglobin.

**Why Each Wrong Option is Incorrect:**

A. Aspirin (salicylic acid) poisoning does not result in a cherry red coloration of the retina. Aspirin primarily affects platelet function and alters the balance between prostaglandins and thromboxanes, leading to increased bleeding tendency.

B. Cyanide poisoning causes methemoglobinemia, which is characterized by reduced oxygen-carrying capacity due to the formation of methemoglobin (MetHb) from hemoglobin. However, this does not result in a cherry red coloration of the retina.

C. Nitrite poisoning leads to methemoglobinemia, similar to cyanide poisoning. Nitrites react with hemoglobin to form methemoglobin, but it does not cause cherry red coloration of the retina.

**Clinical Pearl:**
The cherry red spot is a valuable clinical sign in detecting carbon monoxide poisoning and is crucial for early recognition and appropriate management of the patient. In cases of suspected carbon monoxide poisoning, it is essential to perform a thorough ophthalmologic examination, including the cherry red spot, to support the clinical suspicion and initiate prompt treatment with hyperbaric oxygen therapy.