Which is not a principal feature of osteogenesis imperfecta?

Correct Answer: D

Rationale: Microcephaly is not a typical feature of osteogenesis imperfecta, unlike fragile bones, blue sclerae, and deafness.

Correct Answer: C

Rationale: Pseudohyponatremia occurs due to high plasma lipids or proteins (e.g., myeloma, hypertriglyceridemia) displacing water. Protein-losing enteropathy reduces proteins, not causing pseudohyponatremia.

Correct Answer: D

Rationale: SIADH leads to euvolemic hyponatremia due to excess water retention without volume depletion or expansion, with low serum osmolality.

Correct Answer: D

Rationale: Hypomagnesemia prolongs PR, QRS, and QT intervals due to impaired cardiac repolarization.

Correct Answer: C

Rationale: Renal insufficiency is the most common cause of hyperphosphatemia due to reduced phosphate excretion.