What is the most likely diagnosis for a patient with hypertension and hypercholesterolemia, taking aspirin, ramipril, and simvastatin, presenting with sudden right knee pain, swelling, no fever, and chondrocalcinosis on X-ray?

Pseudogout (Calcium Pyrophosphate Deposition Disease)

The most likely diagnosis is pseudogout (option e), given the acute monoarticular knee pain and swelling with radiographic evidence of chondrocalcinosis in a patient with hypertension and hypercholesterolemia. 1

Key Diagnostic Features Supporting Pseudogout

The presence of chondrocalcinosis on X-ray is the pathognomonic radiographic finding that distinguishes this presentation from other causes of acute monoarthritis. 1 Chondrocalcinosis represents calcium pyrophosphate crystal deposition in fibrocartilage and hyaline cartilage, with the knee menisci being one of the most common target sites. 1

Clinical Presentation Consistent with Pseudogout

• Sudden onset of pain and swelling in a single joint (monoarticular presentation) is characteristic of acute calcium pyrophosphate deposition disease 2
• The knee is the most frequently affected joint in pseudogout, followed by the wrist 1
• Absence of fever does not exclude crystal arthropathy and is common in pseudogout 2
• The patient's age and comorbidities (hypertension, hypercholesterolemia) fit the typical demographic, as pseudogout predominantly affects elderly patients 2

Why Other Diagnoses Are Less Likely

Gouty Arthritis (Option b)

While gout can present similarly with acute monoarticular arthritis, chondrocalcinosis is not a feature of gout. 1 Gout is characterized by monosodium urate crystal deposition, which produces different radiographic findings including erosions with overhanging edges, not chondrocalcinosis. 1

Septic Arthritis (Option d)

The absence of fever makes septic arthritis less likely, though it cannot be completely excluded on clinical grounds alone. 1 The American College of Physicians recommends synovial fluid analysis when clinical judgment indicates diagnostic testing is necessary to differentiate between crystal arthropathy and septic arthritis. 1 However, the presence of chondrocalcinosis strongly points toward pseudogout rather than infection.

Osteoarthritis (Option a)

While osteoarthritis can cause chronic knee pain and may coexist with chondrocalcinosis, the sudden onset of acute pain and swelling is not typical of primary osteoarthritis. 1 The acute inflammatory presentation suggests an active crystal deposition process rather than degenerative joint disease alone.

Reactive Arthritis (Option c)

Reactive arthritis typically follows a genitourinary or gastrointestinal infection and would not produce chondrocalcinosis on radiographs. 1

Recommended Diagnostic Confirmation

Synovial fluid analysis should be performed to confirm the diagnosis by demonstrating calcium pyrophosphate dihydrate crystals, which appear as rhomboid-shaped, weakly positively birefringent crystals under polarized light microscopy. 1 This is particularly important because:

• It definitively confirms crystal arthropathy 1
• It excludes septic arthritis, which can coexist with crystal disease 1
• The American College of Physicians recommends synovial fluid analysis when clinical judgment indicates diagnostic testing is necessary in patients with possible acute gout or pseudogout 1

Clinical Pitfall to Avoid

Do not assume the absence of fever excludes septic arthritis entirely. If there is any clinical concern for infection (particularly in immunocompromised patients or those with prosthetic joints), arthrocentesis with synovial fluid analysis including cell count, Gram stain, and culture must be performed before initiating treatment. 1 Crystal arthropathy and septic arthritis can occasionally coexist.