**Core Concept**
The first external sign of decomposition in a dead body is a result of the breakdown of hemoglobin in the blood, leading to the formation of sulfhemoglobin, which causes a greenish discoloration of the skin. This process is accelerated by the presence of bacteria in the abdominal cavity, particularly in the cecum, which is located in the right iliac fossa.

**Why the Correct Answer is Right**
The correct answer, B. Greenish discoloration over right iliac fossa, is due to the fact that the cecum, a pouch-like structure in the large intestine, is the primary site of bacterial fermentation in the gut. As bacteria begin to break down the body's tissues, they release gases, including hydrogen sulfide, which reacts with hemoglobin to form sulfhemoglobin. This reaction causes a greenish discoloration of the skin, which is most noticeable over the right iliac fossa, where the cecum is located. The greenish discoloration is a visible sign of the decomposition process and can be an important indicator of post-mortem changes.

**Why Each Wrong Option is Incorrect**

* **Option A:** Decomposition of liver and intestine is a more advanced stage of decomposition, occurring after the initial signs of putrefaction have appeared. The liver and intestine are rich in blood and bacteria, but decomposition of these organs is not the first external sign of decomposition.
* **Option C:** Greenish discoloration over dependent parts is a possible sign of decomposition, but it is not the first external sign. Dependent parts of the body, such as the back and buttocks, may become discolored due to the pooling of blood and the breakdown of tissues, but this is not the primary site of decomposition.
* **Option D:** Bloodstained froth from the mouth is a sign of pulmonary edema, which can occur in cases of drowning or other conditions affecting the lungs. It is not a sign of decomposition.

**Clinical Pearl / High-Yield Fact**
The right iliac fossa is a key area to examine in cases of suspected decomposition, as it is the primary site of bacterial fermentation and the first external sign of putrefaction. This knowledge can be useful in forensic medicine, where the identification of post-mortem changes is critical in determining the cause and time of death.

✓ Correct Answer: **B.** Greenish discoloration over right iliac fossa.