Intrarenal foreign bodies are rare and could be iatrogenic through direct penetration, penetration through the gastrointestinal tract (GIT), or retrograde migration from the lower urinary tract. According to their mode of entrance , foreign bodies in the upper urinary tract system were divided into three categories according to Gondos et al., 1955: Direct penetration, penetration through a hole in the nearby GIT, and ascent from the lower urinary tract. [1] 
Sometime , things accidentally left inside the body after surgeries cause retrograde migration.[2]
We present Intraoperatively incidental findings of an intrarenal foreign body during percutaneous nephrolithotomy (PCNL) with no previous urological intervention. However, there is a significant multiple percutaneous liver abscess drainage and endoscopic procedures. This case report of a pigtail
thread rupture, likely resulting from recent hepatic abscess drainage.

A 72-year-old male diabetic with bronchial asthma, post-cholecystectomy, presented with right upper quadrant pain and fever and was admitted due to cholangitis with a large liver abscess. The patient successfully underwent CT-guided drainage by an intervention radiologist. CT guided (Seldinger Tech.) 10F pigtail drainage catheter was inserted in the right hepatic abscess about 9*6 cm in diameter.The staghorn stone is planned to be managed electively for at least six months after complete recovery, clearance of infection, and anticoagulant. It shows an incidental staghorn stone in the pelvis and lower calyx, measuring about 3 cm and 400 HU; only the stone was visible on the CT two months later, an MRI follow-up showed dilated CBD of about 15mm with dilated IHBR. Moreover, there are multiple variable-sized hypointense filling defects along the course of CBD, the largest of which is about 10mm. ERCP was done for the patient with a biliary stent. Eight months after the previous event, the patient electively underwent percutaneous nephrolithotomy. Intraoperatively, the lower calyx
was used to access the kidney while the patient was prone. Dilators were used to expose the renal pelvis. The calculi were first broken up using the pneumatic lithotripter (Swiss Lithoclast, EMS, Le Sentier, Switzerland). With a 20Fr nephroscope, the stone was visualized,and after some fragmentation, an intra-renal foreign body was incidentally found. Forceps removed the foreign body without complication after careful inspection. An antegrade double J stent was inserted, and the procedure was finished

Five days following the procedure, the patient was discharged from the hospital.The foreign substance was discovered during the postoperative examination as a piece of the pigtail thread, most likely from a recent
hepatic abscess drainage. It was hidden by the staghorn stone, explaining its invisibility in pre-operative imaging. One month later, he was scheduled for a follow-up appointment to have the double J stent removed, which was accomplished. The stones were submitted for component analysis. After eight months of monitoring,there was no stone recurrence.

A history of invasive interventions should be obtained before managing
renal stones. The presentation of intrarenal foreign bodies could
mimic nephrolithiasis or urinary tract infections, hydronephrosis, and
urosepsis. There is no standard treatment protocol for retrieval of an
intrarenal foreign body. Treatment can be challenging by retrograde
intrarenal surgery and PCNL.

Gondos B. Foreign body in the left kidney and ureter. J Urol. 1955 Jan;73(1) :35-38.
Upadhyay SP, Zahir M, Al Muttari H, Mallick PN. A rare case of unusual migrated foreign bodies in kidney and their successful extraction using retrograde percutaneous nephrostomy. Qatar Med J. 2015 Apr 15;2015(1).