Membership of the Royal Colleges of Physicians (MRCP) Practice Exam

The presence of positive crystals in joint fluid analysis is a key indicator that can help differentiate between types of arthritis. When considering a patient with knee pain who has crystals present, the type of crystals identified will lead to the most accurate diagnosis.

In this scenario, gouty arthritis, which is caused by monosodium urate crystals due to hyperuricemia, typically presents with acute and severe pain, often affecting the first metatarsophalangeal joint, though it can involve other joints like the knee. If the crystals detected are calcium pyrophosphate dihydrate, this corresponds to what is known as pseudogout, which primarily affects larger joints, commonly the knees, and can mimic the symptoms of gout.

Given the two-day history of knee pain alongside positive crystals, if the diagnosis of pseudogout is made, it indicates the presence of calcium pyrophosphate crystals, which is a classic hallmark of this condition. Pseudogout tends to present acutely and can be mistaken for gout but will have a different underlying crystal pathophysiology.

Thus, in the context of this patient’s symptoms coupled with crystal analysis findings, the diagnosis of pseudogout would be supported by the specifics of the crystalline composition observed.