A 42-year-old man presents to his doctor with complaints of left-sided flank pain radiating into his groin. The pain began suddenly and has increased in intensity. He had a similar problem several years ago, but he did not seek medical attention, and his symptoms resolved spontaneously. He denies fever, chills, nausea, or vomiting. He has had no diarrhea or difficulty urinating. Physical examination is remarkable for tenderness along the left flank and into the groin, but is otherwise normal. There are no inguinal hernias detected on examination. A urinalysis reveals microscopic hematuria without casts. Serum chemistry reveals: Sodium: 143 mEq/L Potassium: 2.4 mEq/L Bicarbonate: 17 mEq/L Chloride: 115 mEq/L Blood urea nitrogen: 12 mg/dL Creatinine: 1.0 mg/dL Glucose: 85 mg/dL Which of the following is MOST likely present in this patient?
High-Yield Explanation
This patient's bicarbonate is lower than the normal range of 22-28 mEq/L. This means that there is less bicarbonate in his blood than usual. If his breathing is normal, then his CO2 level should be about normal. Bicarbonate is produced when CO2 binds to an H2O molecule, which then splits into a H+ ion and a HCO3- (bicarbonate) ion. This reaction is governed by an equilibrium constant propoional to the concentrations of the products multiplied together over the concentrations of the reactants multiplied together. By convention, the "water" concentration is ignored because it is "constant." So, in order for the equilibrium constant to not change value, the H+ concentration must increase if the bicarbonate decreases and the CO2 remains the same. This means that an acidosis is produced. In this case, it is a metabolic acidosis, since there is no indication of respiratory disease in this patient. Neither a normal acid-base status nor a metabolic alkalosis would have a low bicarbonate. Note: The hallmark of metabolic acidosis is decreased HCO3-. Metabolic acidosis are classified by the anion gap, either normal or increased. The anion gap is the difference between readily measured anions and cations. Ref: Cho K.C. (2013). Chapter 21. Electrolyte & Acid-Base Disorders. In M.A. Papadakis, S.J. McPhee, M.W. Rabow (Eds), CURRENT Medical Diagnosis & Treatment 2013.