**Core Concept**
The underlying principle being tested is the diagnosis and management of **pseudogout**, a type of arthritis caused by the deposition of **calcium pyrophosphate dihydrate (CPPD) crystals** in the joint. This condition is characterized by sudden, severe joint pain and swelling, often affecting the knee.

**Why the Correct Answer is Right**
Since CPPD crystals have been identified, the next step is to investigate for potential underlying conditions that may contribute to CPPD deposition, such as hyperparathyroidism, hemochromatosis, or renal disease. However, the options are not provided to explain this in detail.

**Why Each Wrong Option is Incorrect**
**Option A:** Without knowing the specific option, it's hard to comment, but typically, an incorrect option might not directly relate to the diagnosis or management of pseudogout.
**Option B:** Similarly, without the option, it's challenging to provide a specific reason, but it might not address the need to investigate underlying causes or manage the condition appropriately.
**Option C:** This option might be incorrect if it suggests a treatment or investigation that is not directly relevant to pseudogout or CPPD crystal deposition disease.
**Option D:** Without the specific option, it's difficult to say, but it could be incorrect if it does not align with the standard approach to diagnosing or managing pseudogout.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the identification of CPPD crystals is diagnostic of pseudogout, and further investigations should focus on identifying any underlying metabolic or endocrine disorders that may be contributing to the condition.

**Correct Answer:** Correct Answer: D. Radiograph of the affected joint.