**Core Concept:**
The question is asking about a clinical sign seen on chest radiography, specifically the appearance of "cannon balls." This refers to small, rounded, lucent densities (holes) in the lung fields, which can provide diagnostic clues for certain conditions. These are not actual cannon balls but rather radiographic features.

**Why the Correct Answer is Right:**
Cannon balls are indicative of small, well-circumscribed, and densely calcified lymph nodes, typically seen in the mediastinum. In this context, Option D (Calcified mediastinal lymph nodes) is the correct answer. Calcification in lymph nodes is a normal process, particularly in the mediastinum, due to calcification of the lymph node cortex and medulla.

**Why Each Wrong Option is Incorrect:**

A. Calcified pleural plaques (Option A) are different from cannon balls, as they are flat, irregular, and indistinct densities on the pleura.
B. Calcified pulmonary nodules (Option B) are different due to their size, which is typically larger (less than 3 cm) and less well-circumscribed compared to cannon balls.
C. Calcified mediastinal lymph nodes (Option C) are not the same as cannon balls, as the latter is a specific term describing well-circumscribed, dense, and lucent densities in the lung fields.

**Clinical Pearl:**
Cannon balls are a characteristic finding on chest radiography and help clinicians narrow down the differential diagnosis. In the correct context, such as calcified mediastinal lymph nodes, they can be crucial in making a diagnosis or confirming a suspicion. Understanding these radiographic findings is essential in interpreting chest radiographs accurately for diagnosing various medical conditions.