A pregnant women presents for prenatal care. Her past medical history is significant for a severe illness 3 years ago characterized by fatigue, nausea, anorexia, vomiting, jaundice, joint pains, and generalized skin lesions that slowly disappeareD. She has felt well recently. Which lab test should be ordered to investigate the patient’s past illness?

**Core Concept**
The patient's past medical history suggests a multisystem illness characterized by fatigue, nausea, anorexia, vomiting, jaundice, joint pains, and generalized skin lesions, which are all classic symptoms of Systemic Lupus Erythematosus (SLE). SLE is a chronic autoimmune disease that can affect multiple organs and systems in the body.

**Why the Correct Answer is Right**
The patient's symptoms are consistent with a previous episode of SLE, which can present with a wide range of clinical manifestations. The presence of antinuclear antibodies (ANA) is a hallmark of SLE and can be detected in the majority of patients with the disease. Ordering an ANA test would help confirm the diagnosis and guide further management.

**Why Each Wrong Option is Incorrect**
**Option A:** A complete blood count (CBC) would not be useful in diagnosing SLE, as the disease is not primarily characterized by anemia or thrombocytopenia. While a CBC may show some abnormalities in SLE patients, it is not a specific or sensitive test for the disease.

**Option B:** A liver function test (LFT) may show abnormalities in SLE patients, particularly those with liver involvement, but it is not a specific test for the disease. SLE can cause a wide range of liver abnormalities, including hepatitis and cirrhosis.

**Option C:** A urinalysis may show abnormalities in SLE patients, particularly those with renal involvement, but it is not a specific test for the disease. SLE can cause a wide range of renal abnormalities, including glomerulonephritis and nephrotic syndrome.

**Option D:** A rheumatoid factor (RF) test may be positive in some SLE patients, but it is not a specific test for the disease. RF is more commonly associated with rheumatoid arthritis and other autoimmune diseases.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that SLE can present with a wide range of symptoms and can affect multiple organs and systems in the body. A high index of suspicion and a thorough medical history are crucial in diagnosing SLE, particularly in pregnant women who may be at increased risk of developing the disease.

**Correct Answer:** D. Rheumatoid factor (RF) test.

✓ Correct Answer: A. Hepatitis B surface antigen (HBsAg)