**Core Concept**
The question tests the student's ability to diagnose a condition causing intermittent flank pain, hematuria, and urinary frequency, which are classic symptoms of a ureteral calculus. A ureteral calculus, also known as a kidney stone, is a small, hard mineral mass that forms inside the urinary tract and can cause severe pain, bleeding, and obstruction of urine flow.

**Why the Correct Answer is Right**
The patient's symptoms, such as mild pain in the right iliac fossa, increasing during the night, and exhaustion, are consistent with a ureteral calculus. The waveform pattern of pain is characteristic of a ureteral calculus as the stone moves down the ureter. The presence of hematuria (blood in the urine) and mild urinary frequency are also indicative of a ureteral calculus. The urine pH of 5.5 is not particularly helpful in this case, as it can be seen in various conditions. The urine examination revealing plenty of RBCs and 50 WBCs/hpf further supports the diagnosis of a ureteral calculus.

**Why Each Wrong Option is Incorrect**
**Option A:** Glomerulonephritis is a condition that affects the glomeruli of the kidneys, leading to inflammation and damage. It typically presents with symptoms such as hematuria, proteinuria, and hypertension, but not with intermittent flank pain.

**Option B:** Ca-Urinary bladder (bladder cancer) can cause hematuria and urinary frequency, but it is less likely to cause intermittent flank pain. Additionally, bladder cancer is more common in older adults, and the patient's age is not consistent with this diagnosis.

**Option D:** Cystitis is an inflammation of the bladder, often caused by a bacterial infection. It can cause symptoms such as dysuria (painful urination), frequency, and urgency, but not typically intermittent flank pain.

**Clinical Pearl / High-Yield Fact**
A classic teaching point in urology is that a ureteral calculus typically causes pain that radiates from the flank to the groin, known as "flank to groin pain." This is due to the stone's movement down the ureter and its compression of the ureteral wall.

**✓ Correct Answer: C. Ureteral calculus. A ureteral calculus is the most likely diagnosis based on the patient's symptoms and urine examination findings.**