**Core Concept**
Gross hematuria, which is the visible presence of blood in the urine, can occur due to various urinary tract injuries, including those caused by urethral injuries (UI). In this scenario, the patient's history of urethral injury followed by gross hematuria suggests a possible complication of the initial injury.

**Why the Correct Answer is Right**
The most likely cause of gross hematuria in this scenario is a urethral stricture or a bladder neck injury leading to a false passage or a urethral diverticulum. This can occur when the urethral laceration is not properly managed, leading to scar tissue formation and subsequent narrowing of the urethral lumen. The urethral diverticulum can also cause bladder neck contracture, leading to urinary obstruction and hematuria. The mechanism involves the disruption of the urethral epithelial lining, leading to bleeding and potentially, urinary retention.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while a urinary tract infection (UTI) can cause hematuria, it is typically accompanied by symptoms like dysuria, urgency, and fever, which were not mentioned in the scenario. Moreover, UTI-induced hematuria is usually microscopic, not gross.

**Option B:** This option is incorrect because a kidney stone (nephrolithiasis) can cause hematuria, but it would typically be accompanied by severe flank pain and possibly a palpable mass. The patient's history of urethral injury makes this option less likely.

**Option C:** This option is incorrect because a bladder tumor or carcinoma would be an unusual cause of gross hematuria in a young female, especially in the context of a recent urethral injury.

**Clinical Pearl / High-Yield Fact**
It is essential to consider the patient's medical and surgical history when evaluating hematuria, as it can provide crucial clues to the underlying cause. In this scenario, the patient's history of urethral injury is a critical factor in diagnosing the cause of gross hematuria.

**Correct Answer: D.**