A 45-year-old woman with a history of celiac disease and alcoholism is admitted to the ICU. There is no evidence of significant acute or chronic blood loss. Her hematocrit is 30%, hemoglobin is 9 g/L, and mean corpuscular volume is 105 fL. Her serum methylmalonic acid concentration is within normal limits, and her serum homocysteine concentration is elevated. Serum ferritin is within normal limits. Which does this patient most likely has a deficiency of?

Correct Answer: C

Rationale: Elevated homocysteine with normal methylmalonic acid and macrocytic anemia (MCV 105 fL) suggest folic acid deficiency, common in alcoholism and celiac disease, rather than vitamin B12 (cyanocobalamin) deficiency, which would elevate both markers.

Correct Answer: A

Rationale: Kidneys primarily regulate water loss through urine concentration, not water gain, which depends on intake.

Correct Answer: B

Rationale: Loop diuretics block sodium and potassium reabsorption in the loop of Henle, increasing potassium loss in urine.

Correct Answer: B

Rationale: High blood pressure inhibits vasopressin secretion to decrease water reabsorption and lower volume.

Correct Answer: D

Rationale: Angiotensin II triggers thirst, vasoconstriction, and aldosterone release to increase blood volume and pressure.