BCSE - Basic and Clinical Sciences Examination Clinical Nutrition and Metabolism Questions and Answers Flashcards

A critically ill patient on prolonged parenteral nutrition develops hypophosphatemia, hemolytic anemia, and reduced diaphragmatic contractility. Which metabolic mechanism best explains these findings collectively?

Answer: ATP depletion secondary to intracellular phosphate deficiency disrupting energy-dependent cell functions

Refeeding syndrome causes profound hypophosphatemia as glucose infusion drives phosphate intracellularly for ATP synthesis. The resulting intracellular phosphate depletion reduces ATP availability in erythrocytes (causing hemolytic anemia from membrane pump failure) and respiratory muscles (reducing diaphragmatic contractility). While 2,3-DPG depletion also occurs and shifts the oxygen-dissociation curve, the unifying mechanism across all three findings is generalized ATP depletion from phosphate deficiency.

A 58-year-old woman with short bowel syndrome has had 80 cm of terminal ileum resected. She develops steatorrhea, fat-soluble vitamin deficiencies, and hyperoxaluria with recurrent calcium oxalate nephrolithiasis. Which mechanism most directly explains the nephrolithiasis in this context?

Answer: Unabsorbed fatty acids saponify intraluminal calcium, leaving free oxalate available for colonic absorption

Enteric hyperoxaluria in short bowel syndrome results from fat malabsorption: unabsorbed long-chain fatty acids bind intraluminal calcium (saponification), leaving dietary oxalate unbound and free to be absorbed in the colon in large quantities. Normally, calcium binds oxalate in the gut lumen, forming insoluble calcium oxalate that is excreted in stool. The increased colonic oxalate load leads to hyperoxaluria and calcium oxalate nephrolithiasis. Increased intestinal permeability contributes marginally but is not the primary mechanism.

During prolonged starvation beyond 5–7 days, the brain's reliance on ketone bodies instead of glucose is a critical adaptation. Which enzymatic step is the rate-limiting factor that prevents the liver itself from oxidizing ketone bodies it produces?

Answer: Absence of succinyl-CoA:3-oxoacid CoA transferase (SCOT) in hepatic mitochondria

The liver lacks succinyl-CoA:3-oxoacid CoA transferase (SCOT), the enzyme required to convert acetoacetate back to acetoacetyl-CoA for entry into the TCA cycle. This enzymatic absence is precisely why the liver is an obligate ketone producer but not consumer — it cannot 're-activate' ketone bodies for its own oxidation. This ensures that ketone bodies produced in the liver are exported exclusively to peripheral tissues (brain, heart, muscle) during starvation. HMG-CoA synthase is actually upregulated during fasting to promote ketogenesis.

A patient receiving long-term home parenteral nutrition is found to have a plasma selenium level of 45 μg/L (reference: 70–150 μg/L) and presents with proximal muscle weakness and cardiomyopathy. Her formula was compounded without selenium supplementation for 18 months. Which enzymatic function is most compromised by this deficiency?

Answer: Glutathione peroxidase activity, reducing protection against lipid peroxidation in cell membranes

Selenium is an essential component of selenocysteine-containing proteins, most critically glutathione peroxidase (GPx). GPx reduces hydrogen peroxide and lipid hydroperoxides using glutathione as a cofactor, protecting cell membranes from oxidative damage. In selenium deficiency, GPx activity falls markedly, resulting in accumulation of lipid peroxides in cardiac myocyte membranes and skeletal muscle. This is the primary mechanism underlying Keshan disease (dilated cardiomyopathy) and the myopathy seen in TPN-dependent patients. While iodothyronine deiodinases are also selenoproteins and contribute, GPx compromise is the most direct cause of the cardiomyopathy and myopathy in this clinical scenario.

A researcher measures respiratory quotient (RQ) in a patient receiving intravenous lipid emulsion at a rate that exceeds the patient's oxidative capacity. Which metabolic fate of excess fat most likely causes the RQ to rise above the theoretical RQ for fat oxidation (0.71)?

Answer: Excess acetyl-CoA is re-esterified into triglycerides via de novo lipogenesis, a process that consumes NADPH and releases CO₂

When fat infusion exceeds oxidative capacity, excess acetyl-CoA is redirected toward de novo lipogenesis (DNL). DNL is a carbogenic process: fatty acid synthesis from acetyl-CoA and NADPH releases CO₂ (via the malic enzyme shuttle and the pentose phosphate pathway). The net reaction for converting glucose or excess acetyl-CoA to fat produces substantial CO₂ relative to O₂ consumed, raising the measured RQ above 0.71 — and in extreme cases above 1.0 (as seen with carbohydrate-driven lipogenesis, RQ ~8.0 theoretical). The same principle applies when fat overloading drives lipogenic pathways, elevating measured CO₂ production and thus RQ.

A 44-year-old man with alcoholic cirrhosis and hepatic encephalopathy is started on a branched-chain amino acid (BCAA)-enriched enteral formula. The theoretical rationale for BCAA supplementation in this population is most precisely explained by which mechanism?

Answer: BCAAs are predominantly metabolized in skeletal muscle rather than the liver, and their oxidation provides an alternative energy source while competitively reducing aromatic amino acid uptake across the blood-brain barrier

In hepatic encephalopathy, the plasma ratio of aromatic amino acids (AAAs: phenylalanine, tyrosine, tryptophan) to BCAAs (leucine, isoleucine, valine) is markedly elevated due to impaired hepatic AAA catabolism. AAAs and BCAAs compete for the same neutral amino acid transporter (LAT1) at the blood-brain barrier. Excess AAAs enter the brain, where they serve as precursors for false neurotransmitters (octopamine, tyramine) and serotonin, contributing to encephalopathy. BCAAs are catabolized primarily in muscle (not liver), making them safe nitrogen sources. Supplementing BCAAs normalizes the molar ratio, competitively reduces AAA transport into the CNS, and provides fuel for muscle. The Fischer ratio (BCAA:AAA) is the key quantitative framework underlying this therapy.