Step 1 topics → Musculoskeletal, Skin, and Connective Tissue → Pseudogout (CPPD)

Pseudogout (CPPD)

USMLE Step 1 trap: Confuses CPPD crystal morphology and birefringence with monosodium urate crystals. CPPD crystals are rhomboid-shaped and weakly positively birefringent, in contrast to the needle-shaped, negatively birefringent urate crystals of gout.

Pseudogout (CPPD — calcium pyrophosphate deposition disease) is a crystal arthropathy caused by calcium pyrophosphate crystals depositing in joint cartilage and synovial fluid. It mimics gout clinically but is a completely different disease at the crystal level, and USMLE Step 1 loves to exploit exactly that confusion. The classic presentation is an older adult with acute monoarthritis of the knee plus chondrocalcinosis (calcification of cartilage) on X-ray — that X-ray finding is the hallmark that separates CPPD from gout on a clinical vignette.

The exam tests this topic primarily through crystal analysis questions and clinical differentiation from gout. You'll see a synovial fluid analysis result and need to know that CPPD crystals are rhomboid-shaped and weakly positively birefringent — the opposite of the needle-shaped, negatively birefringent urate crystals in gout. Students who memorize 'birefringent = gout' without anchoring the sign (positive vs. negative) will get this wrong every time. Step 1 also tests whether you know which metabolic diseases drive CPPD in younger patients, because that changes your workup entirely.

What makes this concept genuinely tricky is the number of parallel contrasts to gout: different crystal shape, different birefringence sign, different joint, different treatment. Each of those can be independently tested. The management question is a trap — students reaching for allopurinol or probenecid are confusing the underlying chemistry. CPPD has nothing to do with uric acid, so urate-lowering therapy is simply irrelevant here.

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Common misconceptions

Common mistake
Wrong: Pseudogout crystals are needle-shaped and negatively birefringent like gout.
Right: CPPD crystals are rhomboid-shaped and weakly positively birefringent, in contrast to the needle-shaped, negatively birefringent urate crystals of gout.
CPPD and gout crystals are opposite on every property the exam tests. Gout crystals are needle-shaped (long, pointed) and negatively birefringent (yellow when parallel to the polarizer axis). CPPD crystals are rhomboid-shaped (short, rectangular) and weakly positively birefringent (blue when parallel). Build the contrast as a paired table in your mind — if you only memorize one, you will misassign the other on test day.
Common mistake
Wrong: Pseudogout, like gout, most commonly affects the first MTP joint.
Right: Pseudogout most commonly affects the knee (and other large joints), whereas gout classically affects the first MTP joint.
The first MTP joint ('podagra') is the signature of gout, not pseudogout. CPPD preferentially deposits in larger joints with fibrocartilage, and the knee is by far the most classic. When a vignette gives you knee monoarthritis in an older adult, think CPPD first; when it's the big toe, think gout first. Joint location alone won't diagnose it, but it's a strong orienting clue before you get to crystal analysis.
Common mistake
Wrong: Allopurinol or uricosuric agents should be used to treat pseudogout.
Right: Urate-lowering therapy has no role in pseudogout because CPPD crystals contain calcium pyrophosphate, not urate; treatment is symptomatic with NSAIDs, colchicine, or steroids.
Allopurinol inhibits xanthine oxidase to reduce uric acid production, and uricosuric agents increase uric acid excretion — neither has any mechanism relevant to calcium pyrophosphate. CPPD is not a uric acid disorder at all. Treating a CPPD flare is purely symptomatic: NSAIDs, colchicine, or intra-articular/systemic steroids. Reaching for allopurinol here is a category error, like treating a bacterial infection with antivirals.
Common mistake
Gap: Misses metabolic workup (hyperparathyroidism, hemochromatosis, hypomagnesemia) indicated by early-onset pseudogout
CPPD in a young patient should prompt screening for metabolic causes: hyperparathyroidism, hemochromatosis, hypomagnesemia, hypophosphatasia, and Wilson disease.
In an older patient, CPPD is often idiopathic. But when CPPD appears in someone under 55, the exam expects you to hunt for a metabolic cause driving abnormal calcium or pyrophosphate metabolism. The key diseases to screen: hyperparathyroidism (elevated PTH → hypercalcemia), hemochromatosis (iron deposition damages cartilage), hypomagnesemia (magnesium normally inhibits crystal formation), hypophosphatasia (low alkaline phosphatase → pyrophosphate accumulates), and Wilson disease. This is a classic 'what workup do you order?' angle on USMLE Step 1.
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What the exam tests

  1. Know the classic presentation: older adult, acute knee monoarthritis, and chondrocalcinosis (cartilage calcification) visible on plain X-ray — this is the imaging hallmark of CPPD.
  2. Identify CPPD crystals on synovial fluid analysis: rhomboid shape and weakly positive birefringence under polarized light, which is the direct opposite of gout's needle-shaped, negatively birefringent urate crystals.
  3. Recognize that CPPD occurring in a young patient is a red flag for underlying metabolic disease and know the specific conditions to screen for: hyperparathyroidism, hemochromatosis, hypomagnesemia, hypophosphatasia, and Wilson disease.
  4. Select the correct management for an acute CPPD flare — NSAIDs, colchicine, or corticosteroids — and know why allopurinol and uricosuric agents have no role in this disease.

Can you avoid these mistakes?

A 72-year-old woman presents with a swollen, red, painful knee. Synovial fluid shows weakly positively birefringent rhomboid crystals. X-ray shows calcification within the knee cartilage. What is the diagnosis, and what X-ray finding confirms it?
You see a synovial fluid report: 'needle-shaped crystals, negatively birefringent.' A second report says: 'rhomboid crystals, weakly positively birefringent.' Which report corresponds to gout and which to CPPD — and what joint would you expect each patient to be complaining about?
A 38-year-old man is diagnosed with CPPD after presenting with acute knee swelling. His age prompts you to screen for secondary causes. Name at least four metabolic conditions that should be on your workup list and briefly explain why each can cause CPPD.
Your attending asks you to prescribe allopurinol for a patient with a confirmed CPPD flare. Why is this incorrect, and what would you prescribe instead?

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