**Core Concept**
The investigation of choice in the early phase of renal transplant is crucial for monitoring graft function, detecting potential complications, and guiding management decisions. Renal function tests, imaging studies, and biopsies are essential components of post-transplant care.

**Why the Correct Answer is Right**
The correct answer is **B. Ultrasound**. Ultrasound is the investigation of choice in the early phase of renal transplant due to its non-invasive nature, lack of radiation exposure, and ability to detect potential complications such as graft thrombosis, hydronephrosis, or perirenal fluid collections. Ultrasound can also guide percutaneous biopsy, which is essential for histological assessment of graft function and potential rejection.

**Why Each Wrong Option is Incorrect**
**Option A:** Blood tests, such as serum creatinine and urea levels, are essential in monitoring graft function but are not the investigation of choice in the early phase of renal transplant.

**Option C:** CT angiography is not the investigation of choice in the early phase of renal transplant due to its high radiation exposure and lack of sensitivity in detecting perirenal fluid collections.

**Clinical Pearl / High-Yield Fact**
In the early phase of renal transplant, regular ultrasound monitoring is essential to detect potential complications, and a high index of suspicion is required to diagnose and manage graft dysfunction promptly.

**Correct Answer: B. Ultrasound**