American Board of Internal Medicine (ABIM) Certification Practice Exam

Which type of kidney stones are radiolucent on plain radiographs but become visible on CT scans or ultrasounds?

1. Calcium oxalate stones

2. Struvite stones

3. Cystine stones

4. Uric acid stones

The correct answer is: Uric acid stones

Uric acid stones are radiolucent on plain radiographs, meaning they do not typically show up on X-ray imaging due to their low calcium content. This characteristic makes it challenging to diagnose uric acid stones using conventional radiographic methods. However, these stones can be detected using computed tomography (CT) scans or ultrasounds, which are more sensitive imaging modalities. CT scans provide detailed cross-sectional images that can identify the stones even if they are not visible on standard X-rays. In contrast, other types of stones, such as calcium oxalate and struvite stones, are generally radiopaque and can be seen on plain radiographs due to their higher calcium content. Cystine stones can also be detected on X-rays, though they may not be as easily identified in some cases. Uric acid stones remain unique in that they do not appear on regular X-rays but are clearly visible with other imaging techniques. This distinction is important in clinical practice for the proper diagnosis and management of patients with kidney stone disease.