**Core Concept**
Carbon monoxide (CO) poisoning occurs when carbon monoxide binds to hemoglobin, forming carboxyhemoglobin (COHb), which reduces oxygen delivery to tissues. This condition is a leading cause of poisoning deaths worldwide.

**Why the Correct Answer is Right**
The diagnosis of CO poisoning post-mortem is primarily based on the presence of carboxyhemoglobin in the blood, which is detectable through various methods such as spectrophotometry or co-oximetry. The level of COHb is directly proportional to the concentration of CO in the blood, making it a reliable indicator of CO exposure. High levels of COHb (> 40%) are typically associated with severe CO poisoning, which can be fatal.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is likely incorrect because it does not specify a direct method for diagnosing CO poisoning post-mortem. While a history of exposure to CO may be obtained, it is not a reliable method for confirming CO poisoning.

**Option B:** This option is incorrect because CO poisoning does not typically cause a decrease in blood pH. In fact, CO poisoning often leads to a compensatory increase in respiratory rate, which can result in respiratory alkalosis.

**Option C:** This option is likely incorrect because while methemoglobinemia is a type of hemoglobinopathy, it is not directly related to CO poisoning. Methemoglobinemia is caused by an abnormal amount of methemoglobin in the blood, which can be due to various factors such as certain medications or toxins.

**Clinical Pearl / High-Yield Fact**
The classic "cherry-red" discoloration of the skin and mucous membranes in CO poisoning is due to the increased concentration of carboxyhemoglobin in the blood, which gives tissues a bright red or pinkish hue.

**Correct Answer:** D