Correct Answer: A. Putty kidney
Putty kidney (also called "granular kidney" or "putty-like kidney") is a chronic sequela of renal tuberculosis characterized by caseous necrosis and fibrosis of the renal parenchyma, resulting in a non-functioning, shrunken kidney with a putty-like consistency on gross pathology. The radiological hallmark is a calcified, shrunken kidney with a characteristic "putty" or "granular" appearance on plain X-ray and CT. The clinical presentation of sterile pyuria (pus in urine without bacterial growth) is pathognomonic for renal TB, as the causative organism Mycobacterium tuberculosis is acid-fast and requires special staining; routine urine culture remains sterile. In India, where TB prevalence remains high, renal TB is a common cause of sterile pyuria. The X-ray findings showing a calcified, non-functioning kidney with loss of normal renal contour are diagnostic. Putty kidney represents end-stage renal TB and typically requires nephrectomy as definitive management. The combination of sterile pyuria + calcified shrunken kidney on imaging is virtually diagnostic of putty kidney.
Why the other options are wrong
B. Psoas calcification — Psoas calcification (psoas abscess with calcification) is a sequela of spinal TB, not renal TB. While both are TB-related, psoas calcification presents with back pain and flank swelling, not sterile pyuria. The X-ray would show calcification in the psoas muscle region, not within the kidney itself. This is a common distractor because both are TB-related radiological findings in India. C. Nephrocalcinosis — Nephrocalcinosis refers to calcium deposition within the renal parenchyma (medullary or cortical), typically seen in hypercalcemia, hyperparathyroidism, renal tubular acidosis, or medullary sponge kidney. It presents with bilateral symmetrical calcification within the kidney substance, not a shrunken calcified kidney. Nephrocalcinosis does not cause sterile pyuria and is not associated with TB. D. Staghorn calculus — Staghorn calculus is a large struvite or calcium phosphate stone that fills the renal pelvis and branches into the calyces, resembling a stag's horn on imaging. While it can cause recurrent UTIs, it typically presents with bacterial pyuria (not sterile), and the urine culture is positive. The radiological appearance is of a branching calculus within the collecting system, not a diffusely calcified shrunken kidney. Staghorn calculi are associated with chronic infection but not specifically with TB.
High-Yield Facts
- Sterile pyuria is the hallmark of renal TB; routine urine culture is negative because M. tuberculosis requires special media (Löwenstein-Jensen).
- Putty kidney is end-stage renal TB with caseous necrosis, fibrosis, and calcification resulting in a non-functioning, shrunken kidney.
- X-ray findings in putty kidney: calcified, shrunken kidney with loss of normal renal contour; may show 'moth-eaten' calyces.
- Nephrectomy is the definitive treatment for putty kidney; medical TB therapy alone cannot restore function to a putty kidney.
- Renal TB prevalence in India remains significant; always suspect TB in patients with sterile pyuria and calcified kidney on imaging.
Mnemonics
PUTTY KIDNEY = TB Pus in urine (sterile pyuria) + Unfunctioning + Tuberculosis + Therapy-resistant + Yield to nephrectomy. Remember: putty kidney is TB until proven otherwise when you see sterile pyuria + calcified kidney. Sterile Pyuria Differential (STAB) Spinal TB (psoas abscess), Tuberculosis (renal), Acute glomerulonephritis, Bladder stones. When you see sterile pyuria in India, think TB first.
NBE Trap
NBE pairs "calcified kidney" with nephrocalcinosis to trap students who confuse bilateral symmetrical parenchymal calcification (nephrocalcinosis) with a shrunken, calcified non-functioning kidney (putty kidney). The key discriminator is the clinical context: sterile pyuria + TB history points to putty kidney, not nephrocalcinosis.
Clinical Pearl
In Indian clinical practice, any patient presenting with sterile pyuria should raise immediate suspicion for renal TB. A calcified, shrunken kidney on plain X-ray or CT in this context is virtually diagnostic of putty kidney. Early recognition is critical because putty kidney is non-salvageable and requires nephrectomy; delaying diagnosis risks complications like hypertension and renal failure in the contralateral kidney.
_Reference: Robbins Ch. 20 (Kidney); Harrison Ch. 335 (Tuberculosis); Park's Textbook of Preventive and Social Medicine (TB epidemiology in India)_