Urolithiasis with stone formation: case reportA 79-year-old woman developed urolithiasis with stone formation during treatment with cotrimoxazole for recurrent urinary tract infection [dose not stated; duration of treatment to reaction onset not stated].The woman, who had multiple comorbidities, developed urosepsis due to obstructing ureterolithiasis in 2018. She underwent percutaneous nephrostomy tube insertion and treatment with 14 days course of oral cotrimoxazole [trimethoprim/ sulfamethoxazole]. She received the treatment for recurrent urinary tract infections by multiple organisms including Klebsiella pneumoniae, Proteus mirabilis and extended spectrum beta lactamase (ESBL) resistant Escherichia coli. She underwent a cystolitholopaxy and mini-percutaneous nephrolithotomy for nephrolithiasis in 2020, however due to the significant stone burden, she was unable to achieve a stone-free state. Stone analysis revealed stone composition of magnesium salts, calcium salts and ammonium acid urate. Her primary care physician intermittently administered multiple additional courses of cotrimoxazole for recurrent urinary tract infections. In 2021 at the age of 79 years, she presented with urosepsis due to an obstructing stone confirmed on CT. A ureteral stent was placed and 14 days of culture-directed cotrimoxazole was administered. She underwent bilateral, staged ureteroscopy and cystolitholopaxy due to her comorbidities. Between the two ureteroscopy procedures, she had received 21-day course of additional prophylactic cotrimoxazole. Stone analysis with Fourier transform infrared (FTIR) spectroscopy for stone obtained during the second ureteroscopy were sent and showed that the brown stones were composed of sulfamethoxazole. A diagnosis of single episode of urolithiasis with obstructing stone formation in kidney attributed to cotrimoxazole was made.The woman was discharged to her skilled nursing facility with nitrofurantoin prophylaxis and had recovered well.