Correct Answer: Statements 1, 3 and 4

Vitamin D supplementation is a common therapy for vitamin D deficiency and related hypocalcemic/hyperparathyroid states. Each statement is analyzed below against the authoritative correct answer, which is option "1, 3 and 4".

Statement-wise Analysis

• Statement 1: Serum calcium levels improve earlier than serum PTH levels — Correct.

  With vitamin D repletion, intestinal calcium absorption increases rapidly, and serum calcium normalizes within days to a few weeks. Secondary hyperparathyroidism, however, resolves much more slowly because chronically stimulated parathyroid glands take weeks to months to down-regulate PTH secretion and parathyroid cellular hyperplasia to regress. Hence, a normalizing serum calcium with a still-elevated PTH is a well-recognized pattern during vitamin D replacement, confirming this statement.

• Statement 2: Patients should be closely observed with serial vitamin D levels in blood as vitamin D toxicity occurs frequently — Incorrect.

  Vitamin D toxicity is actually uncommon at standard replacement/maintenance doses used clinically; it is seen mainly with prolonged, very high-dose (often inadvertent or over-the-counter) intake. Routine practice does not mandate serial 25-hydroxyvitamin D monitoring in every patient — monitoring is instead focused on serum calcium (and occasionally 24-hour urinary calcium) to detect hypercalcemia, which is the earlier and more clinically relevant marker of toxicity. Because the statement overstates the frequency of toxicity and misidentifies the recommended monitoring parameter, it is false.

• Statement 3: Vitamin D supplementation should always be in conjunction with calcium supplementation — Correct.

  Vitamin D's principal action is to enhance intestinal calcium absorption; without adequate elemental calcium substrate, vitamin D alone cannot correct hypocalcemia or effectively suppress secondary hyperparathyroidism, and its skeletal benefits are blunted. Standard treatment protocols for vitamin D deficiency and for bone health (e.g., in osteoporosis, osteomalacia, and hypoparathyroidism-related states) therefore pair vitamin D with concurrent calcium supplementation, making this statement correct.

• Statement 4: Nephrolithiasis is a known complication — Correct.

  Excess vitamin D activity raises intestinal calcium absorption and can produce hypercalcemia and hypercalciuria. Elevated urinary calcium excretion is a well-established risk factor for calcium-oxalate/calcium-phosphate stone formation, making nephrolithiasis a recognized complication of vitamin D supplementation, particularly with excessive dosing.

Why the Other Options Are Wrong

• 1, 2 and 3 — incorrectly includes statement 2 (toxicity is not frequent and serial vitamin D monitoring is not the standard) while correctly including 1 and 3.
• 1, 2 and 4 — again wrongly includes statement 2 and omits statement 3, which is a valid and standard clinical recommendation.
• 2, 3 and 4 — incorrectly includes statement 2 and omits statement 1, which describes a true and well-documented biochemical sequence (calcium normalizing before PTH).

Conclusion

Statements 1, 3, and 4 are true, while statement 2 is false. The correct answer is therefore "1, 3 and 4".