**Core Concept**
Putrefaction is the process of breakdown of tissue, particularly after death, due to microbial action. This process starts in the organs with the highest concentration of bacteria and the lowest concentration of oxygen.

**Why the Correct Answer is Right**
The first organ to start putrefaction is the **intestine**. This is because the intestine has a high concentration of bacteria, which are the primary agents of putrefaction. Additionally, the intestine has a low oxygen tension, which creates an environment conducive to microbial growth and putrefaction. The bacteria in the intestine break down the intestinal contents, leading to the production of gases such as hydrogen sulfide, which is responsible for the characteristic odor of putrefaction.

**Why Each Wrong Option is Incorrect**
**Option A:** The stomach is not the first organ to start putrefaction because it has a higher concentration of acid, which inhibits microbial growth.
**Option B:** The liver is not the first organ to start putrefaction because it has a relatively low concentration of bacteria compared to the intestine.
**Option C:** The spleen is not the first organ to start putrefaction because it is not a primary site of bacterial colonization.

**Clinical Pearl / High-Yield Fact**
The order of putrefaction is often remembered as "B.O.M.B." - Brain, Ovaries, Midgut (intestine), and Muscles (in that order). This sequence is useful for medical examiners to estimate the post-mortem interval.

**Correct Answer: D. Intestine**