**Core Concept**
The question is testing the knowledge of the sequence of putrefaction in the human body. Putrefaction is the process of breakdown of tissues by microorganisms, leading to the release of foul-smelling gases and fluids. This process is influenced by factors such as oxygen availability, pH, and the presence of microorganisms.

**Why the Correct Answer is Right**
The first internal organ to undergo putrefaction is the stomach. This is because the stomach has a high concentration of bacteria, such as Clostridium perfringens, which are anaerobic and can thrive in the absence of oxygen. The stomach also has a low pH, which favors the growth of these bacteria. As these bacteria break down the stomach contents, they release gases such as hydrogen sulfide, which is responsible for the characteristic odor of putrefaction.

**Why Each Wrong Option is Incorrect**
* **Option A:** The small intestine is not the first internal organ to undergo putrefaction, as it has a lower concentration of bacteria compared to the stomach.
* **Option B:** The liver is not the first internal organ to undergo putrefaction, as it is protected by a layer of peritoneum and has a lower concentration of bacteria.
* **Option C:** The kidneys are not the first internal organ to undergo putrefaction, as they are relatively sterile and have a lower concentration of bacteria.

**Clinical Pearl / High-Yield Fact**
The sequence of putrefaction can be remembered using the mnemonic "SOFT": Stomach, Ovaries, Femur, and Thoracic cavity, which are the first organs to undergo putrefaction in the human body.

**Correct Answer:** D.