American Board of Internal Medicine (ABIM) Certification Practice Exam

What type of nephrolithiasis is associated with hexagonal stones?

1. Calcium Phosphate

2. Struvite

3. Cystine

4. Uric Acid

The correct answer is: Cystine

Hexagonal stones are characteristic of cystine nephrolithiasis. Cystine stones form due to a genetic disorder known as cystinuria, which leads to the excessive excretion of the amino acid cystine in the urine. These stones typically appear as hexagonal crystals under polarized light. Patients with cystinuria have difficulty reabsorbing cystine in the renal tubules, resulting in supersaturation of urine with cystine, which crystallizes to form these distinct stones. In contrast, the other types of stones listed have different compositions and appearances. Calcium phosphate stones are commonly rhomboid and do not exhibit the hexagonal form. Struvite stones are often associated with urinary infections and are typically feather-like or staghorn in shape, linked to the presence of ammonium and phosphate. Uric acid stones, while they can be formed in certain metabolic conditions, tend to appear as rhomboid or rosette crystals but not hexagonal. Thus, in the context of nephrolithiasis, the presence of hexagonal stones specifically indicates a diagnosis of cystine stones.