## Core Concept
The lower triangular space of the arm, also known as the triangular space or inferior triangular space, is an anatomical region in the arm. It is one of the intermuscular spaces of the arm and is significant for the passage of neurovascular structures. The boundaries of this space are typically defined by specific muscles.

## Why the Correct Answer is Right
The lower triangular space is bounded by the **teres major muscle** (superiorly), **long head of triceps brachii** (medially), and **humerus** (laterally). This space allows for the passage of the **circumflex scapular artery** and the **radial nerve**. Understanding these boundaries is crucial for diagnosing and managing injuries or conditions affecting this region.

## Why Each Wrong Option is Incorrect
* **Option A:** This option is actually one of the boundaries, so it cannot be the correct answer to the question about which one does NOT form a boundary.
* **Option B:** Similarly, if this is a boundary, it doesn't fit the description of the exception.
* **Option C:** If this option describes a boundary of the lower triangular space, then it is not the correct answer to the question.
* **Option D:** The **latissimus dorsi** muscle does not form a boundary of the lower triangular space; instead, the boundaries are formed by the **teres major**, the **long head of triceps brachii**, and the **humerus**. The latissimus dorsi is more related to the posterior chest wall and does not directly contribute to the boundaries of this specific space.

## Clinical Pearl / High-Yield Fact
A key clinical correlation is that the radial nerve, which runs through this space, can be injured in fractures of the humerus or in penetrating trauma, leading to **radial nerve palsy**. This condition can result in wrist drop and loss of extension of the wrist and fingers.

## Correct Answer: D. latissimus dorsi.