Patient Information
Age: 67 years
Gender: M, self-identified
Race/Ethnicity: unspecified
Site of Care: office

History
Reason for Visit/Chief Concern: "I'm tired all the time and my back hurts."
History of Present Illness:
• 4-month history of fatigue and low back pain
• 3-kg (6.6-lb) weight loss
• no vomiting, diarrhea, increased thirst, or confusion
Past Medical History:
• hypertension
Medications:
• amlodipine; no diuretics
Allergies:
• none
Family History:
• noncontributory
Psychosocial History:
• does not drink alcohol

Physical Examination
Temp 36.8°C (98.2°F) · Pulse 82/min · Resp 14/min · BP 132/78 mm Hg · O₂ Sat 97% on RA · Ht 175 cm · Wt 72 kg · BMI 23.5 kg/m²
• Appearance: pale; mucous membranes moist
• Back: point tenderness over L2
• Extremities: no edema
• Neurologic: alert and oriented; no focal deficits

Diagnostic Studies
Blood: hemoglobin 9.6 g/dL · hematocrit 29% · smear with rouleaux
Serum (central laboratory analyzer): Na⁺ 132 mEq/L · K⁺ 4.4 mEq/L · Cl⁻ 99 mEq/L · HCO₃⁻ 24 mEq/L · urea nitrogen 20 mg/dL · creatinine 1.1 mg/dL · glucose 90 mg/dL · calcium 9.0 mg/dL
Total protein 13.2 g/dL · albumin 3.0 g/dL
Measured serum osmolality 290 mOsm/kg H₂O (N 275–295)
Na⁺ measured on a blood gas analyzer (direct ion-selective electrode): 139 mEq/L

Question: Which of the following best explains the serum sodium reported by the central laboratory?

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