The correct answer is B. Serum calcium.
A serum calcium measurement is most likely to identify the cause of this patient’s symptoms. This young woman has recurrent attacks of acute calcium pyrophosphate (CPP) crystal arthritis (pseudogout). Clues to the diagnosis include acute and recurrent self-limited monoarticular arthritis, chondrocalcinosis on radiograph, and, most tellingly, identification of CPP crystals under polarizing microscopy. This condition usually affects the elderly; therefore, evidence of acute CPP crystal arthritis in a young person should always prompt an investigation for secondary causes. Secondary causes include hyperparathyroidism, hypothyroidism, hypophosphatasia, hypomagnesemia, and hemochromatosis. This patient has recent-onset signs and symptoms suggestive of hypercalcemia and hyperparathyroidism, including abdominal discomfort, constipation, and weakness. Therefore, measuring the serum calcium is appropriate (and serum parathyroid hormone, if hypercalcemia is present), because this patient appears to have CPP deposition due to hyperparathyroidism.
Anti–cyclic citrullinated peptide antibodies are useful in the diagnosis of rheumatoid arthritis (RA). RA typically presents with the insidious onset of symmetric arthritis in the hands and feet along with morning stiffness. Monoarthritis is a rare presenting feature of RA, but it is usually followed by development of the symmetric arthritis.
Checking serum creatine kinase would not be a first-line test for this patient. Although muscle disease is associated with weakness, myositis would not account for many of the other manifestations, including the monoarticular arthritis, radiographic chondrocalcinosis, and synovial fluid crystals.
Serum urate would be an appropriate laboratory study to measure if gout was suspected, but the synovial fluid analysis is consistent with acute CPP crystal arthritis rather than gout, in which negatively birefringent crystals are seen. Gout also more typically affects the metatarsophalangeal joints, the mid foot, ankles, and knees, although the finger joints may be affected in postmenopausal women.
- Evidence of acute calcium pyrophosphate crystal arthritis (pseudogout) in a young person should always prompt an investigation for secondary causes such as hyperparathyroidism, hypothyroidism, hypophosphatasia, hypomagnesemia, and hemochromatosis.