**Core Concept**
The underlying principle in this question is the evaluation of lower urinary tract anomalies in infants, particularly after a urinary tract infection (UTI). Vesicoureteral reflux (VUR) is a common condition that can lead to recurrent UTIs, and it is often diagnosed using imaging studies.

**Why the Correct Answer is Right**
The most appropriate investigation for evaluating the lower urinary tract in this patient is a voiding cystourethrogram (VCUG). This imaging study involves the introduction of a contrast agent into the bladder, followed by the patient's natural micturition process. The VCUG provides a dynamic assessment of the lower urinary tract, allowing for the detection of VUR, which is a common cause of UTIs in infants. The VCUG is particularly useful in this case because the ultrasound examination was normal, and the VCUG can provide more detailed information about the lower urinary tract.

**Why Each Wrong Option is Incorrect**
**Option A:** Ultrasonography is a useful initial imaging study for evaluating the kidneys and urinary tract, but it may not detect VUR or other lower urinary tract anomalies. **Option B:** Cystoscopy is an invasive procedure that may be used to evaluate the upper urinary tract, but it is not the most appropriate initial investigation for evaluating the lower urinary tract in this patient. **Option C:** Renal scintigraphy can provide information about renal function and the excretion of contrast agents, but it is not the most sensitive test for detecting VUR or other lower urinary tract anomalies.

**Clinical Pearl / High-Yield Fact**
In infants with a history of UTI, a normal ultrasound examination does not rule out VUR or other lower urinary tract anomalies. A VCUG should be considered as part of the diagnostic evaluation to ensure that these conditions are not missed.

**Correct Answer:** C.