**Core Concept**
The question is testing the understanding of the relationship between acute urinary tract infections (UIs) and potential renal pathologies. Specifically, it is evaluating the student's knowledge of the possible complications that can arise after a UI, such as the development of gross hematuria.

**Why the Correct Answer is Right**
Gross hematuria 3 days after an attack of UI is likely due to a renal pathology known as pyelonephritis with papillary necrosis. Pyelonephritis is an infection of the renal pelvis and parenchyma, which can lead to papillary necrosis, a condition characterized by the necrosis of the renal papillae. This can cause gross hematuria due to the shedding of necrotic papillae into the urine.

**Why Each Wrong Option is Incorrect**
**Option A:** Acute glomerulonephritis is a possible complication of a UI, but it typically presents with microscopic hematuria and proteinuria rather than gross hematuria.

**Option B:** Renal calculi (kidney stones) can cause hematuria, but they are not directly related to the development of UI.

**Option C:** Acute nephritic syndrome is a condition characterized by hematuria, proteinuria, and hypertension, but it is not typically associated with gross hematuria 3 days after a UI.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that pyelonephritis with papillary necrosis can be a complication of UI, especially in patients with diabetes or those who are immunocompromised.

**Correct Answer:** C.