Differential Diagnosis
- Single most likely diagnosis:
- Systemic Lupus Erythematosus (SLE): The patient's symptoms, including the butterfly-shaped facial rash, diffuse joint pain, and dark-colored urine, are classic for SLE. The disease is known for its multi-system involvement and can affect the kidneys, joints, and skin, among other systems. The presence of a facial rash and joint pain, combined with potential renal involvement suggested by the dark urine, makes SLE a strong consideration.
- Other Likely diagnoses:
- Henoch-Schönlein Purpura (HSP): Although less likely given the specific pattern of the rash and the lack of mention of abdominal pain or purpura, HSP could be considered due to the joint pain and potential renal involvement.
- Rheumatoid Arthritis (RA): While RA can cause joint pain, the presence of a specific facial rash and dark-colored urine makes it less likely than SLE. However, it remains a consideration in the differential for joint pain.
- Do Not Miss (ddxs that may not be likely, but would be deadly if missed.):
- Vasculitis (e.g., Granulomatosis with Polyangiitis): Although less common, vasculitis can present with a wide range of symptoms including renal involvement and skin rashes. Missing this diagnosis could lead to severe consequences, including organ failure.
- Goodpasture Syndrome: This rare autoimmune disease can cause renal failure and pulmonary hemorrhage, presenting with hematuria (which could cause dark urine) and is a critical diagnosis not to miss due to its severity.
- Rare diagnoses:
- Mixed Connective Tissue Disease: A rare autoimmune disease that overlaps with SLE, RA, and scleroderma, which could potentially explain some of the patient's symptoms.
- Sjögren's Syndrome: An autoimmune disorder that primarily affects the glands that produce tears and saliva but can also have systemic manifestations, including renal involvement and skin rashes.
Laboratory Finding Most Likely to be Decreased
Given the clinical presentation suggestive of SLE, the laboratory finding most likely to be decreased is the Serum C3 concentration (D). In SLE, the complement system, particularly C3 and C4, is often activated, leading to decreased levels of these complement components due to consumption. This is a common finding in active SLE, especially when there is renal involvement.