**Question:** Rapidly progressive glomerulonephritis is characterised by:

**Core Concept:** Rapidly progressive glomerulonephritis (RPGN) is a clinical syndrome characterized by a rapid decline in kidney function, typically associated with inflammation (glomerulonephritis) of the glomeruli. It is a serious condition that can lead to end-stage renal disease if left untreated.

**Why the Correct Answer is Right:** RPGN is characterized by the following factors:

1. **Option A (Increased proteinuria):** Proteinuria is a significant symptom of glomerulonephritis, with increased protein loss in urine. Rapidly progressive glomerulonephritis can cause a rapid increase in proteinuria due to the inflammation and damage to the glomerular basement membrane.
2. **Option B (Reduced urine output):** As the glomerular filtration rate (GFR) decreases, urine output is reduced, reflecting the decline in kidney function. In RPGN, the rapid progression causes a more pronounced decrease in GFR compared to other forms of glomerulonephritis.
3. **Option C (Hypocomplementemic uraemia):** Hypocomplementemic uraemia is a subset of RPGN characterized by low complement levels. Complement activation plays a crucial role in the immune response, and its deficiency contributes to the rapid progression of the disease.
4. **Option D (Rapid decline in kidney function):** RPGN presents with a rapid decline in kidney function, leading to end-stage renal disease if left untreated. This rapid progression distinguishes RPGN from other forms of glomerulonephritis.

**Why Each Wrong Option is Incorrect:**

1. **Option A (Low complement levels):** While some forms of RPGN are indeed associated with low complement levels (hypocomplementemic uraemia), not all cases of RPGN exhibit this feature.
2. **Option B (Proteinuria without albuminuria):** Proteinuria and albuminuria are interrelated, as albumin is a larger protein than other proteins. Hence, an absence of albuminuria in the presence of proteinuria could be indicative of a different type of glomerulonephritis.
3. **Option D (No immune complex deposition):** Immune complex deposition is a characteristic feature of RPGN, as it involves the formation of immune complexes that deposit in the glomeruli, leading to inflammation and damage.
4. **Option E (Absence of immune complex deposition):** Immune complex deposition is a crucial feature of RPGN, as mentioned above. Absence of this feature would point towards a different type of glomerulonephritis.

**Clinical Pearl:**

Understanding the underlying pathology, such as immune complex deposition, is crucial in diagnosing and managing RPGN. Rapidly progressive nephrotic syndrome is a critical clinical feature that prompts further investigation and evaluation for the underlying cause, as prompt treatment is crucial to prevent end-stage renal disease.