**Core Concept**
The submandibular gland is a salivary gland located in the submandibular triangle, and its stones are often large and radio-opaque. Detection of submandibular calculi requires a radiographic projection that optimally visualizes this area.

**Why the Correct Answer is Right**
The lower true occlusal (LTO) projection is the best suited for detecting submandibular calculi. This is because the LTO projection provides a clear view of the submandibular gland and its duct, allowing for the identification of stones within the gland or its duct. The LTO projection involves tilting the occlusal plane 15-20° from the vertical plane, which optimizes the visualization of the submandibular gland and its surrounding structures.

**Why Each Wrong Option is Incorrect**
**Option B:** The lower topographic occlusal projection is not ideal for detecting submandibular calculi as it does not provide a clear view of the submandibular gland and its duct.

**Option C:** The lower oblique occlusal projection may not provide an optimal view of the submandibular gland and its duct, as the angle of the projection may not be sufficient to visualize these structures clearly.

**Option D:** The vertex occlusal projection is not suitable for detecting submandibular calculi as it provides a frontal view of the skull, which does not allow for the optimal visualization of the submandibular gland and its duct.

**Clinical Pearl / High-Yield Fact**
When ordering radiographs for suspected salivary gland calculi, it is essential to specify the projection that will provide the best view of the affected gland. In this case, the lower true occlusal projection is the best choice for detecting submandibular calculi.

**✓ Correct Answer: A. Lower true occlusal**