Okay, let's tackle this question. The patient is a 65-year-old woman with cervical spondylosis, but the main issue here is her elevated serum calcium level at 12.5 mg%. That's hypercalcemia. Normal serum calcium is around 8.5-10.2 mg/dl, so this is significantly high. The hematocrit and kidney function tests (KFT) are normal, but phosphate is low at 2.3 mg/dl. Low phosphate along with hypercalcemia is a classic finding in hyperparathyroidism, especially primary.

The question is asking for the first investigation to be done. The options aren't listed here, but common initial tests for hypercalcemia include measuring PTH levels. If PTH is elevated, it points to primary hyperparathyroidism. If it's low or normal, then other causes like malignancy, vitamin D toxicity, or other conditions need to be considered.

Other possible tests might include a PTHrP (parathyroid hormone-related protein) level for malignancy, vitamin D levels, and imaging studies. But the first step is usually to check PTH. The low phosphate supports the idea of increased bone resorption due to PTH.

Now, looking at the options if they were present, the correct answer would be the one that suggests measuring PTH. The other options might include things like urine calcium, X-rays, or other tests, but PTH is the key initial test.

I need to make sure the explanation covers the core concept of hypercalcemia, the role of PTH, why PTH is the first test, and why other options are wrong. Also, the clinical pearl should highlight that PTH is the first test to distinguish primary hyperparathyroidism from other causes.

**Core Concept** Hypercalcemia (serum calcium >10.2 mg/dl) requires urgent evaluation of parathyroid hormone (PTH) levels to differentiate primary hyperparathyroidism from malignancy or other causes. Hypophosphatemia often accompanies PTH-mediated bone resorption.

**Why the Correct Answer is Right** The first investigation should be **serum intact PTH**. Elevated PTH confirms primary hyperparathyroidism (common in elderly women), while low/normal PTH indicates malignancy (e.g., PTHrP-secreting tumors) or other causes (e.g., vitamin D toxicity). Normal hematocrit and KFT (kidney function tests) argue against chronic kidney disease, a common cause of secondary hyperparathyroidism.

**Why Each Wrong Option is Incorrect**
**Option A:** *Urinary calcium* – Useful for distinguishing renal vs. bone resorption but is **not first-line** in hypercalcemia.
**Option B:** *Serum vitamin D* – Elevated 25(OH)D or 1,25(OH)₂D may suggest vitamin D toxicity, but PTH testing is priority.
**Option C:** *Chest X-ray* – Imaging is secondary to biochemical evaluation and misses occult malignancies.

**Clinical Pearl / High-Yield Fact** Remember **"PHI"** for hypercalcemia workup: **P**TH (first test), **H**ematocrit (to assess hydration), **I**onized calcium. Primary hyper

✓ Correct Answer: A. Serum PTH