A 64-year-old man comes to the emergency department due to severe abdominal pain, nausea, and vomiting. He has a history of hypertension, myocardial infarction, and systolic heart failure. His blood pressure is 100/60 mm Hg, pulse is 116/min and irregular, and respirations are 24/min. Examination shows a soft, mildly distended, and tender abdomen. Laboratory results are as follows: CT scan of the abdomen reveals distal ileal wall thickening and lack of enhancement with intravenous contrast. Decreased activity of which of the following enzymes best explains the acid-base disorder in this patient?

Question

A 64-year-old man comes to the emergency department due to severe abdominal pain, nausea, and vomiting.  He has a history of hypertension, myocardial infarction, and systolic heart failure.  His blood pressure is 100/60 mm Hg, pulse is 116/min and irregular, and respirations are 24/min.  Examination shows a soft, mildly distended, and tender abdomen.  Laboratory results are as follows:  CT scan of the abdomen reveals distal ileal wall thickening and lack of enhancement with intravenous contrast.  Decreased activity of which of the following enzymes best explains the acid-base disorder in this patient?

Quizplus · Accepted Answer

11ec160b_36b3_1567_9249_796b720c1e14_MD0001_00 This patient has acute mesenteric ischemia, with inadequate delivery of oxygen to intestinal tissues. During glycolysis, the concentration of oxygen in the tissues affects the metabolic fate of pyruvate. In the presence of oxygen, pyruvate is preferentially converted to acetyl coenzyme A by pyruvate dehydrogenase in the mitochondrial matrix. Acetyl coenzyme A then undergoes oxidation in the citric acid (TCA) cycle. However, under hypoxic conditions, intracellular accumulation of NADH inhibits pyruvate dehydrogenase. As a result, increased amounts of pyruvate are converted to lactate by lactate dehydrogenase (Choice B), which regenerates NAD⁺ from NADH (allowing for limited ATP production via anaerobic glycolysis). With significant tissue ischemia, lactate begins to accumulate in the circulation and can lead to lactic acidosis. Patients with lactic (or other metabolic) acidosis will typically hyperventilate to eliminate CO₂ and induce a compensatory respiratory alkalosis. (Choices A and E) 2-phosphoglycerate is converted to phosphoenolpyruvate by the enzyme enolase. Phosphoenolpyruvate is subsequently metabolized to pyruvate by pyruvate kinase. These reactions are performed under both aerobic and anaerobic conditions. (Choice C) Pyruvate can also be converted to oxaloacetate by pyruvate carboxylase. This enzyme is involved in regeneration of glucose from pyruvate by the process of gluconeogenesis. Educational objective: In the presence of oxygen, pyruvate produced during glycolysis is converted by pyruvate dehydrogenase to acetyl coenzyme A, which is subsequently metabolized by oxidative phosphorylation. However, if inadequate oxygen is present in the tissues, pyruvate is converted to lactate by lactate dehydrogenase, leading to lactic acidosis. __________ References: Clinical factors and outcomes in patients with acute mesenteric ischemia in the emergency department. (http://www.ncbi.nlm.nih.gov/pubmed/16038369)

A 64-Year-Old Man Comes to the Emergency Department Due to Severe