**Core Concept**
The question tests knowledge of thoracic anatomy, specifically the relationship between the ribs, intercostal spaces, and internal organs. Understanding the **thoracic cage** and its contents is crucial for diagnosing and managing traumatic injuries.

**Why the Correct Answer is Right**
Given the location of the stab wound between the 9th and 10th ribs along the left posterior axillary line, the first organ to be pierced would likely be the **spleen**. This is because the spleen is located in the upper left region of the abdomen, protected by the 9th to 11th ribs. A wound in this area could easily penetrate the abdominal cavity and reach the spleen.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because the liver is primarily located in the right upper quadrant of the abdomen.
**Option B:** Incorrect as the stomach, while located in the upper left quadrant, is not the first organ to be pierced by a wound in this specific location.
**Option D:** Incorrect because the lung, although in the thoracic cavity, would not be the first organ pierced by a wound that penetrates to a depth of about 5 cm in this location, given the ribcage protection and the spleen's position.

**Clinical Pearl / High-Yield Fact**
In traumatic injuries, the location and depth of a wound are critical in determining which organs are at risk. The **spleen** is particularly vulnerable due to its location and relatively fragile nature.

**Correct Answer:** D. spleen