Surgery MCQs Q2

A 16-year-old boy presents with recurrent episodes of renal colic. Urinalysis shows increased excretion of lysine, arginine, ornithine, and cystine. The disorder follows an autosomal recessive inheritance pattern. Management includes urinary alkalinization and penicillamine therapy.

Which type of renal stone is most likely responsible?

A. Cysteine

B. Calcium oxalate

C. Uric acid

D. Triple phosphate

E. Pure oxalate

✅ Correct Answer:

A. Cysteine.

🔍 Decoding the Stem

• 💡 CLUE 1: Lysine, Arginine, Ornithine, Cystine in urine → These are COLA amino acids (Suggests Cystinuria).
• 💡 CLUE 2: Autosomal recessive → Confirms inherited proximal tubular defect.
• 💡 CLUE 3: Urinary alkalinization + penicillamine → Specific therapy for cystine stone dissolution and prevention.

Why Cysteine Stone is Correct

Cystinuria is caused by a defect in renal tubular reabsorption of COLA amino acids.
Cystine is poorly soluble in acidic urine → precipitates → cystine stones.
Stones are: Hard, Radiopaque (faintly), and classically form hexagonal crystals on urine microscopy.
Treatment: High fluid intake, urinary alkalinization (↑ solubility), and Penicillamine (binds cystine → forms soluble complexes).

Why Other Options Are Incorrect

B. Calcium oxalate
Most common overall stone.
Not associated with COLA aminoaciduria. Seen in hypercalciuria, hyperoxaluria.

C. Uric acid
Associated with hyperuricemia and gout.
Radiolucent. Treated with alkalinization but not penicillamine.

D. Triple phosphate (Struvite)
Infection stones caused by urease-producing bacteria.
Associated with large staghorn calculi.

E. Pure oxalate
Related to hyperoxaluria.
Not linked to an amino acid transport defect.

🧠 High-Yield Points

COLA amino acids → Think cystinuria
Inheritance → Autosomal recessive
Microscopy → Hexagonal crystals in urine
Management → Alkalinize urine + Penicillamine

📖 Read More: Click here for full topic: Renal Stones & Pathology

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💡 Clinical Challenge / Follow-Up:

The patient's sibling is now being evaluated for similar symptoms. A specific qualitative screening test is ordered for their urine.

Your Challenge:

1. What classic crystal shape is seen on urine microscopy for this condition?
2. What specific chemical screening test is used to detect this substance in the urine?

Show Answer

✅ Answer:

1. Hexagonal crystals are classically seen on urine microscopy for cystine stones.
2. The Cyanide-nitroprusside test is used as a qualitative screening test (it detects the presence of sulfhydryl groups).