A 19-year-old man presents with coughing up blood and decreased urine output. On examination, he has normal air entry, and heart sounds. There is no skin rash or lymph nodes and his abdomen is normal. CXR reveals patchy infiltrates in both lower lobes and his creatinine is elevated. Urinalysis is positive for red cells and protein. A renal biopsy reveals autoantibodies to basement membranes. Which of the following is the most likely diagnosis?

**Core Concept**
The underlying principle being tested is the diagnosis of autoimmune disorders affecting the lungs and kidneys, specifically those involving the basement membranes. This condition is characterized by the presence of autoantibodies against the glomerular and alveolar basement membranes.

**Why the Correct Answer is Right**
Given the clinical presentation of coughing up blood (hemoptysis) and decreased urine output, along with the laboratory findings of elevated creatinine, hematuria, and proteinuria, and the presence of autoantibodies to basement membranes on renal biopsy, the most likely diagnosis is Goodpasture's syndrome. This condition is caused by the formation of autoantibodies against the type IV collagen in the basement membranes of the lungs and kidneys.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it does not typically involve autoantibodies to basement membranes.
**Option B:** Incorrect as it is more commonly associated with skin and joint manifestations.
**Option C:** Incorrect because, although it can affect the kidneys, it does not typically cause the specific pattern of autoantibody formation seen in this patient.

**Clinical Pearl / High-Yield Fact**
Goodpasture's syndrome is a classic example of an autoimmune disease that can present with both pulmonary and renal manifestations, and the presence of autoantibodies to basement membranes is pathognomonic.

**Correct Answer:** D. Goodpasture's syndrome.

✓ Correct Answer: C. Goodpasture's syndrome