**Core Concept:** Ureterosigmoidostomy is a surgical procedure in which the ureter is anastomosed to the sigmoid colon, allowing urine to drain into the bowel. It is typically performed in patients with severe urological or anorectal disease, where a normal urinary tract drainage is not feasible.

**Why the Correct Answer is Right:** Electrolyte imbalances are common complications after gastrointestinal tract surgery, including ureterosigmoidostomy. The rectus colon contains chloride-rich bicarbonate-deficient fluid from the gastrointestinal tract, which can lead to a gain in chloride ions and a loss of bicarbonate ions. This can result in an imbalance of electrolytes, specifically hypochloraemic, hypokalaemic, and hypomagnesaemic hyponatraemic acidosis. Hypochloraemic hyponatraemia occurs due to excessive sodium loss in the urine, leading to low serum sodium levels. Hypokalaemic acidosis occurs because potassium is lost in urine, causing acidosis. Hypomagnesaemic hypocalciuria occurs due to the loss of magnesium in urine, leading to low serum magnesium levels and increased calcium excretion, respectively.

**Why Each Wrong Option is Incorrect:**

A. This option refers to hypokalaemic alkalosis, which is not a complication of ureterosigmoidostomy. Hypokalaemic alkalosis occurs due to the gain of bicarbonate from the gastrointestinal tract, which is not the case in ureterosigmoidostomy.

B. This option refers to hypokalaemic acidosis, which is not a complication of ureterosigmoidostomy. As explained above, hypokalaemic acidosis occurs due to potassium loss in urine, not relevant to ureterosigmoidostomy.

C. This option refers to hypomagnesaemic hypocalciuria, which is not a complication of ureterosigmoidostomy. Hypomagnesaemic hypocalciuria occurs due to the loss of magnesium in urine, not relevant to ureterosigmoidostomy.

D. This option refers to hypokalaemic alkalosis, which is not a complication of ureterosigmoidostomy. Hypokalaemic alkalosis occurs due to the gain of bicarbonate from the gastrointestinal tract, not relevant to ureterosigmoidostomy.

**Clinical Pearl:** Careful monitoring of electrolyte levels is essential after gastrointestinal tract surgeries, including ureterosigmoidostomy, to detect and treat potential electrolyte imbalances promptly to prevent complications and optimize patient outcomes.