D) Hexagonal crystals that are faintly radiopaque on radiography

This patient has cystinuria, a hereditary defect in proximal convoluted tubule reabsorption of cystine, ornithine, lysine, and arginine. Cystine stones are poorly soluble and precipitate out at low pH. Urine alkalinization, low sodium diet, and chelating agents such as penicillamine are used in treatment.

Answer choice A: Coffin lid shaped stones that are radiopaque on radiography, is incorrect. This describes magnesium ammonium phosphate stones, which are caused by urease positive infections (e.g., Proteus mirabilis). These stones often form staghorn calculi.

Answer choice B: Diamond shaped stones that are radiolucent on radiography, is incorrect. This describes uric acid crystals, which are more common in patients with hyperuricemia, gout, and tumor lysis syndrome.

Answer choice C: Envelope shaped stones that are radiopaque on radiography, is incorrect. This describes calcium oxalate stones, the most common type of kidney stone. These stones can result from hypocitraturia and ethylene glycol ingestion.

Answer choice E: Muddy brown casts, is incorrect. This describes dead renal tubular cells found in acute tubular necrosis (ATN). ATN is most often due to ischemia or nephrotoxic agents such as aminoglycosides and iodinated radiocontrast agents.

Key Learning Point

Cystinuria is characterized by hexagonal crystals in the urine and is caused by defective proximal convoluted tubule reabsorption of cystine, ornithine, lysine, and arginine.