**Core Concept**
Periapical radiographs are a crucial diagnostic tool in endodontic practice, providing valuable information about the internal and external anatomy of teeth. These radiographs are particularly useful for assessing the length of root canals, which is essential for successful endodontic treatment.

**Why the Correct Answer is Right**
Periapical radiographs are ideal for measuring the length of root canals because they offer a clear, two-dimensional representation of the tooth's internal anatomy. By analyzing the radiograph, dentists can estimate the length of the root canal and determine the optimal working length for endodontic instruments. This is particularly important because over-instrumentation can cause damage to the surrounding bone and tissues, while under-instrumentation can lead to persistent infection.

**Why Each Wrong Option is Incorrect**
* **Option B:** While periapical radiographs can provide indirect evidence of pulp infection, they are not the most reliable method for assessing the presence of infection. Clinical signs and symptoms, such as pain and sensitivity, are more indicative of pulp infection.
* **Option C:** Periapical radiographs are not capable of assessing the vitality of the pulp. Vitality testing, such as thermal or electrical testing, is a more reliable method for determining pulp vitality.
* **Option D:** Although periapical radiographs can provide some information about pulp infection and pulp vitality, they are not useful for assessing these aspects directly. Therefore, option D is incorrect.

**Clinical Pearl / High-Yield Fact**
When using periapical radiographs to assess the length of root canals, it's essential to remember that the radiographic length of the canal may be shorter than the actual length due to the presence of anatomical variations, such as canal curvatures or bifurcations.

**✓ Correct Answer: A. Length of the root canal**