## **Core Concept**
The patient's symptoms of hemoptysis, facial puffiness, microscopic hematuria, and increased serum creatinine suggest a systemic condition affecting multiple organ systems, particularly the kidneys and lungs. This presentation is highly suggestive of **Goodpasture's syndrome** or **anti-glomerular basement membrane (anti-GBM) disease**, which is characterized by the presence of autoantibodies against the glomerular and alveolar basement membranes.

## **Why the Correct Answer is Right**
The correct approach to investigating this patient would involve confirming the suspected diagnosis of Goodpasture's syndrome. This condition is associated with **anti-GBM antibodies**, which are pathogenic and directly cause damage to the glomeruli in the kidneys and the alveoli in the lungs, leading to the patient's symptoms. Testing for **anti-GBM antibodies** is crucial for diagnosis.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While a chest X-ray might show abnormalities such as pulmonary infiltrates or nodules, it does not specifically diagnose Goodpasture's syndrome.
- **Option B:** Although renal biopsy can provide supportive evidence by showing characteristic linear immunofluorescence along the glomerular basement membrane, it is not the first step in investigation and does not directly test for the pathogenic antibodies.
- **Option C:** Testing for **ANCA (Anti-Neutrophil Cytoplasmic Antibodies)** is relevant in the differential diagnosis of vasculitides, such as granulomatosis with polyangiitis (GPA, formerly known as Wegener's granulomatosis), which can also present with hemoptysis and renal involvement but is not the primary test for Goodpasture's syndrome.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that Goodpasture's syndrome is one of the few autoimmune diseases that presents with **linear** immunofluorescence on renal biopsy, unlike the **pauci-immune** pattern seen in ANCA-associated vasculitides. Rapid diagnosis and treatment are critical to prevent irreversible organ damage.

## **Correct Answer: D. Anti-GBM antibodies.**