What is the differential diagnosis for a patient presenting with left knee pain, a history of psoriasis and hypertension, currently taking lisinopril (angiotensin-converting enzyme inhibitor) and hydrochlorothiazide (diuretic) combination, and dexamethasone (corticosteroid), with a family history of osteoarthritis, colon cancer, and stroke, who has a history of smoking and alcohol use, and has a positive arthrocentesis test with synovial fluid analysis showing leukocytosis with a high percentage of polymorphonuclear cells (PMN) and negatively birefringent crystals?

Differential Diagnosis for Left Knee Pain

Single Most Likely Diagnosis

• Pseudogout (Calcium Pyrophosphate Deposition Disease, CPPD): The presence of negatively birefringent crystals in the synovial fluid, yellow and cloudy appearance, and a history of psoriasis (which can be associated with CPPD) make this diagnosis the most likely. The patient's age and the acute presentation of knee pain also support this diagnosis.

Other Likely Diagnoses

• Gout: Although the crystals are negatively birefringent (which is more characteristic of pseudogout), gout cannot be ruled out entirely without further testing. The patient's history of hypertension and use of hydrochlorothiazide (a diuretic that can increase uric acid levels) also increases the risk of gout.
• Psoriatic Arthritis: Given the patient's history of psoriasis, psoriatic arthritis is a possible diagnosis. However, the presence of crystals in the synovial fluid and the acute presentation make pseudogout more likely.
• Septic Arthritis: Although the patient's symptoms and lab results do not strongly suggest septic arthritis, the high white blood cell count in the synovial fluid (100,000 cells/mm^3) and the presence of 75% PMN cells require consideration of this diagnosis.

Do Not Miss Diagnoses

• Septic Arthritis (infectious): As mentioned earlier, the high white blood cell count and percentage of PMN cells in the synovial fluid necessitate consideration of septic arthritis, which is a medical emergency requiring prompt treatment.
• Lyme Arthritis: Although less likely, Lyme arthritis can present with acute knee pain and should be considered in patients with exposure to ticks, especially if they live in or have traveled to endemic areas.

Rare Diagnoses

• Hemochromatosis-associated Arthropathy: This condition can cause joint pain and is associated with calcium pyrophosphate dihydrate (CPPD) crystal deposition. However, it is rare and would require further testing to diagnose.
• Ochronotic Arthropathy: This condition is associated with alkaptonuria and can cause joint pain and crystal deposition. However, it is extremely rare and would require specific diagnostic testing.