A 60-year-old man has had intermittent pain in his right great toe for the past 2 years. Joint aspiration and crystal analysis shows thin, tapered, needle shaped intracellular crystals that are strongly negatively birefringent. Radiograph demonstrates joint space narrowing of the 1st metatarsophalangeal (MTP) joint with medial soft tissue swelling. What is the most likely cause of this condition?

Monosodium urate crystal deposition

Calcium pyrophosphate deposition

A 62-year-old man comes to the physician because of a 1-day history of dull pain and stiffness of the right knee. He takes chlorthalidone for hypertension. Physical examination of the right knee shows a large effusion and mild erythema; range of motion is limited by pain. Arthrocentesis of right knee yields a cloudy aspirate. Gram stain is negative. Analysis of the synovial fluid shows a leukocyte count of 15,000/mm3 and 55% neutrophils. Microscopic examination of the synovial fluid under polarized light shows positively birefringent rhomboid crystals. Further evaluation of this patient is most likely to show which of the following findings?

Calcification of the meniscal cartilage

Chalky nodules on the external ear

Expression of human leukocyte antigen-B27

Thickening of the synovia at the metacarpophalangeal joints

Elevation of serum uric acid concentration

A 65-year-old man with chronic myelogenous leukemia comes to the physician because of severe pain and swelling in both knees for the past day. He finished a cycle of chemotherapy 1 week ago. His temperature is 37.4°C (99.4°F). Physical examination shows swelling and erythema of both knees and the base of his left big toe. Laboratory studies show: Leukocyte count 13,000/mm3 Serum Creatinine 2.2 mg/dL Calcium 8.2 mg/dL Phosphorus 7.2 mg/dL Arthrocentesis of the involved joints is most likely to show which of the following?

Gram-positive cocci in clusters

A 30-year-old man with a BMI of 33.7 kg/m2 presents with severe pain in his right great toe that began this morning. He had a few beers last night at a friend's party but otherwise has had no recent dietary changes. On examination, the right great toe appears swollen, warm, red, and tender to touch. Joint aspiration is performed. What will examination of the fluid most likely reveal?

Needle-shaped, negatively birefringent crystals on polarized light

Rhomboid-shaped, positively birefringent crystals on polarized light

A 45-year-old woman presents to the clinic for a routine examination. She has a chronic history of systemic lupus erythematosus, diagnosed at age 27. Medications include hydroxychloroquine and low-dose prednisone. She has had no recent flare-ups and is compliant with her medication. Anticardiolipin and anti-beta-2 glycoprotein-1 antibodies are negative, and she has had no history of thrombi or emboli. Physical examination is normal except for mild bilateral tenderness and swelling of the knees. Creatinine and GFR are normal. Which of the following is the next best step in management to monitor disease activity?

Reduce dosage and taper off hydroxychloroquine

Arthrocentesis and synovial fluid analysis

A 33-year-old woman presents to her primary care physician for gradually worsening pain in both wrists that began several months ago. The pain originally did not bother her, but it has recently begun to affect her daily functioning. She states that the early morning stiffness in her hands is severe and has made it difficult to tend to her rose garden. She occasionally takes ibuprofen for the pain, but she says this does not really help. Her medical history is significant for diabetes mellitus and major depressive disorder. She is currently taking insulin, sertraline, and a daily multivitamin. The vital signs include: blood pressure 126/84 mm Hg, heart rate 82/min, and temperature 37.0°C (98.6°F). On physical exam, her wrists and metacarpophalangeal joints are swollen, tender, erythematous, and warm to the touch. There are no nodules or vasculitic lesions. Which of the following antibodies would be most specific to this patient’s condition?

Anti-cyclic citrullinated peptide

A 26-year-old woman presents with 8 weeks of symmetric polyarthritis affecting hands, wrists, and feet with 90 minutes of morning stiffness. RF negative, anti-CCP negative, ANA 1:160 (homogeneous), ESR 45 mm/hr. She has no rash, oral ulcers, or systemic symptoms. Radiographs show soft tissue swelling without erosions. She desires pregnancy within the year. Apply the most appropriate initial management considering her reproductive plans.

Start combination hydroxychloroquine and sulfasalazine for adequate disease control

Defer DMARD therapy until after pregnancy to avoid any fetal risk

Initiate sulfasalazine given safety profile in pregnancy

Begin hydroxychloroquine as pregnancy-compatible DMARD

Start methotrexate with strict contraception counseling; switch when ready to conceive

Crystal arthropathies (gout, pseudogout) — USMLE Lesson

Crystal Arthropathies - An Overview

Inflammatory arthritis from endogenous crystal deposition in/around joints.
Gout: Monosodium Urate (MSU) crystals.
- Needle-shaped, strongly negatively birefringent (yellow when parallel to light).
Pseudogout: Calcium Pyrophosphate Dihydrate (CPPD) crystals.
- Rhomboid-shaped, weakly positively birefringent (blue when parallel). 📌 Mnemonic: Blue = Positive = Pseudogout.

⭐ Gout most commonly affects the first metatarsophalangeal (MTP) joint (podagra).

Gout (MSU) - The King's Complaint

Pathophysiology: Hyperuricemia (serum urate > 6.8 mg/dL) leads to precipitation of monosodium urate (MSU) crystals in joints and soft tissues, triggering an intense inflammatory response.
Presentation: Sudden, severe monoarticular pain, erythema, and swelling.
- Podagra: Inflammation of the 1st metatarsophalangeal (MTP) joint is the classic initial attack (~50% of cases).
- Chronic Gout: Can lead to tophi (visible or palpable urate deposits) and destructive arthropathy.
Diagnosis:
- Arthrocentesis: Gold standard. Synovial fluid reveals needle-shaped, negatively birefringent crystals under polarized light.
- 📌 Crystals are yellow when parallel to the slow ray of the compensator.

⭐ Serum uric acid levels can be normal or low during an acute flare; do not rule out gout based on a normal level. Arthrocentesis is definitive.

Gout vs. Pseudogout: Crystal Microscopy & Birefringence

Pseudogout (CPPD) - The Great Pretender

Deposition of Calcium Pyrophosphate Dihydrate ($Ca_2P_2O_7 \cdot 2H_2O$) crystals in joints & soft tissue.
Acute inflammatory monoarthritis, often mimicking gout. Knee is the most common joint (>50%), followed by the wrist.
Arthrocentesis:
- Rhomboid-shaped crystals.
- Positively birefringent (blue when parallel to light). 📌 Blue = Positive, Parallel.
X-ray: Chondrocalcinosis (linear calcification of cartilage).

⭐ Look for underlying metabolic disease: hemochromatosis, hyperparathyroidism, and hypothyroidism.

Tx: NSAIDs, colchicine, or intra-articular steroids for acute flares.

Crystal Showdown - Gout vs. Pseudogout

Feature	Gout	Pseudogout (CPPD)
Crystal	Monosodium Urate (MSU)	Calcium Pyrophosphate Dihydrate
Shape	Needle-shaped	Rhomboid-shaped (coffin-lid)
Birefringence	Negatively birefringent	Positively birefringent
Color (Compensated Polarized Light)	Yellow when parallel	Blue when parallel
Common Joint	1st Metatarsophalangeal (Podagra)	Knee, wrist
X-ray	Punched-out erosions, tophi	Chondrocalcinosis (cartilage calcification)

Chronic Mgt (Gout): Allopurinol, Febuxostat (Xanthine Oxidase Inhibitors).

⭐ High-Yield: Pseudogout is often associated with underlying metabolic conditions like Hemochromatosis, Hyperparathyroidism, and Hypothyroidism (the "3 H's").

📌 Mnemonic (Birefringence):

Yellow = Parallel (Gout) → "Yella Fella is Parallel"
Blue = Parallel (Pseudogout) → "Blue Positively Parallel"

High‑Yield Points - ⚡ Biggest Takeaways

Gout stems from monosodium urate crystals: needle-shaped and negatively birefringent.

Pseudogout involves calcium pyrophosphate (CPPD) crystals: rhomboid-shaped and positively birefringent.

Arthrocentesis (joint fluid analysis) is the definitive diagnostic test for both.

Classic acute gout often presents as podagra (inflammation of the great toe).

Treat acute flares with NSAIDs, colchicine, or steroids.

Allopurinol is for chronic gout prevention, not for acute attacks.

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