Question

A dense persistent nephrogram may be seen in all of the following except:

• A. Acute ureteral obstruction
• B. Systemic hypertension
• C. Severe hydronephrosis
• D. Dehydration

Hint:

Acute obstruction, hypertension-related slow flow, and dehydration all concentrate contrast; chronic severe hydronephrosis instead damages the kidney and weakens the nephrogram.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
A dense persistent nephrogram is a contrast-enhanced kidney outline on urography or CT that stays unusually dense and lasts longer than normal, because contrast is held up inside the tubules instead of being washed out quickly.

Step 2: Key Concept:
Any condition that slows the flow of filtrate through the tubules while the kidney is still filtering will cause the contrast within it to stay concentrated for longer, producing a dense, persistent nephrogram. This is seen with acute tubular back-pressure (as in fresh obstruction), reduced renal blood flow with preserved tubular reabsorption (as in dehydration or hypotension), and slow flow states such as renovascular hypertension.

Step 3: Detailed Explanation:
In acute ureteral obstruction, the sudden rise in pressure within the tubules slows the passage of filtrate, so contrast stays concentrated in the kidney and the nephrogram becomes dense and persists, so option (A) is a recognised cause.
In dehydration, reduced renal blood flow combined with increased water reabsorption concentrates the contrast in the tubules, again giving a dense persistent nephrogram, so option (D) is a recognised cause.
In systemic (renovascular) hypertension, slow blood flow to the affected kidney lets contrast linger and become more concentrated over time, which is why a dense, delayed nephrogram is actually a classic urographic sign used to suggest renal artery stenosis, so option (B) is a recognised cause, not the exception.
In severe, longstanding hydronephrosis, the kidney has already lost a large part of its functioning tissue because of chronic pressure damage to the cortex. With less functioning parenchyma left, the kidney filters and concentrates contrast poorly, so the nephrogram in severe chronic hydronephrosis is typically thin and faint, not dense, unlike the acute obstructive nephrogram of option (A).

Step 4: Final Answer:
Severe hydronephrosis, because of chronic cortical damage and poor renal function, is the one that does NOT typically give a dense persistent nephrogram.
\[ \boxed{\text{Severe hydronephrosis}} \]
Note: the source paper for this question left its answer key marked with a question mark, so no key was actually stated. This answer is worked out from first principles of nephrogram physiology.