When parathyroid hormone levels fall after parathyroidectomy, the bones rich in calcium begin to remineralize rapidly. This shifts calcium from the blood into bone, causing a fall in serum calcium known as hungry bone syndrome. The primary and most clinically important electrolyte disturbance is hypocalcemia, which can produce symptoms like tingling, muscle cramps, tetany, and even seizures. While calcium, phosphate, and magnesium can all be affected and levels of phosphate and magnesium may also drop as minerals move into bone, the defining and most characteristic disturbance in this scenario is low calcium. Hyperkalemia doesn’t fit the mechanism, and although hypophosphatemia or hypomagnesemia can occur, they are secondary to the dominant issue of hypocalcemia.

Normal anion gap metabolic acidosis happens when bicarbonate is lost or retained with a compensatory rise in chloride, so the measured anion gap stays within the normal range. A classic example is bicarbonate loss through the gastrointestinal tract, such as from diarrhea, where stool bicarbonate is excreted and chloride increases to maintain electroneutrality, producing a hyperchloremic, normal-gap acidosis. The other conditions listed raise the anion gap because they introduce extra unmeasured anions into the blood: lactate in lactic acidosis, ketone bodies in diabetic ketoacidosis, and various retained acids in uremia. Therefore, diarrhea is the cause that results in normal anion gap metabolic acidosis.

Urine anion gap reflects how much ammonium (NH4+) the kidneys are excreting to counteract metabolic acidosis. In metabolic acidosis, the kidneys should boost NH4+ excretion, and this NH4+ is balanced in urine by accompanying chloride. When NH4+ excretion is high, chloride in urine rises and the urine anion gap tends to be negative. If NH4+ excretion is impaired, the chloride balance is reduced and the urine anion gap becomes positive. Hyperkalemia points to reduced aldosterone action or resistance, which characterizes Type 4 RTA. In this form, diminished aldosterone effect lowers NH4+ production and excretion, giving a positive urine anion gap, while potassium tends to be elevated because of reduced distal potassium secretion. Distal RTA can also show a positive urine anion gap, but it typically presents with hypokalemia rather than hyperkalemia. Proximal RTA usually preserves ammonium excretion and tends to have a negative urine anion gap with hypokalemia. Diuretic-induced acidosis can cause a positive gap, but the accompanying hyperkalemia specifically points toward aldosterone-related (Type 4) RTA. So the combination of metabolic acidosis, a positive urine anion gap, and hyperkalemia is most consistent with Type 4 RTA.

Extraglomerular hematuria comes from bleeding sources outside the glomerulus, inside the urinary tract. The classic example among the options is a kidney stone, which can irritate the lining of the kidney, calyces, or ureter as it moves, leading to visible blood in the urine. This type of bleeding is typically non-glomerular in origin and often presents with painless hematuria, sometimes accompanied by colicky flank pain. In contrast, glomerulonephritis involves the filtering units of the kidney themselves, so the urine often shows dysmorphic red blood cells and sometimes red cell casts, reflecting glomerular injury. Nephrotic syndrome is a glomerular condition characterized by heavy protein loss, edema, and hypoalbuminemia, with hematuria not being the defining feature. Renal cell carcinoma can also cause hematuria if the tumor bleeds into the collecting system, but the most characteristic extraglomerular source listed is a kidney stone.