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

A critically ill patient with acute respiratory distress syndrome (ARDS) is receiving enteral nutrition. The respiratory quotient (RQ) is measured at 1.3. Which of the following interventions is MOST appropriate to address the metabolic cause of this elevated RQ?

Answer: Reduce total carbohydrate calories and increase fat-to-carbohydrate ratio in the feeding formula

An RQ >1.0 indicates net lipogenesis — the patient is receiving more carbohydrate than can be oxidized, resulting in de novo fat synthesis and excess CO2 production. This worsens respiratory status in ARDS patients. Reducing carbohydrate intake and shifting to a higher fat-to-carbohydrate ratio lowers CO2 production, reduces the ventilatory burden, and brings RQ back toward 0.7–1.0. Increasing protein or total calories does not address the carbohydrate overfeeding, and switching to PN is not indicated solely for an elevated RQ.

A 58-year-old patient with short bowel syndrome (SBS) has 80 cm of jejunum remaining with an intact ileocecal valve and colon. Which of the following adaptations is MOST critical to monitor for in the colon of this patient over the following 12–24 months?

Answer: Renal oxalate stones due to increased colonic oxalate absorption from fatty acid soap formation

In SBS patients with an intact colon, unabsorbed fat in the colon binds calcium, leaving free oxalate to be absorbed — a phenomenon called enteric hyperoxaluria. This dramatically increases urinary oxalate and the risk of calcium oxalate nephrolithiasis. While colonic fermentation of carbohydrates to SCFAs does occur and is nutritionally beneficial, the clinically critical complication to monitor is oxalate nephropathy, which can lead to permanent renal damage if dietary oxalate is not restricted and calcium supplementation is not optimized.

During the adaptive response to prolonged starvation (>5 days), the brain shifts from exclusive glucose dependence to utilizing ketone bodies. Which enzymatic step represents the rate-limiting transition that allows hepatic ketogenesis to become the dominant fuel export pathway?

Answer: Upregulation of malonyl-CoA decarboxylase, lowering malonyl-CoA levels and relieving inhibition of carnitine palmitoyltransferase I (CPT-I)

The pivotal gate controlling ketogenesis is CPT-I, which transports long-chain fatty acyl groups into the mitochondria for beta-oxidation. Malonyl-CoA is a potent allosteric inhibitor of CPT-I. During starvation, falling insulin and rising glucagon suppress acetyl-CoA carboxylase activity, decreasing malonyl-CoA. Malonyl-CoA decarboxylase further degrades residual malonyl-CoA. The resulting relief of CPT-I inhibition dramatically increases mitochondrial fatty acid entry and acetyl-CoA generation, which overwhelms the TCA cycle capacity and is shunted into ketone body synthesis via mitochondrial HMG-CoA synthase.

A hospitalized patient develops refeeding syndrome 48 hours after aggressive parenteral nutrition initiation following prolonged malnutrition. Laboratory values show: phosphate 1.1 mg/dL, magnesium 1.3 mg/dL, potassium 2.9 mEq/L, glucose 210 mg/dL. Which underlying mechanism MOST directly explains why thiamine deficiency becomes life-threatening in this specific metabolic context?

Answer: Thiamine is required as a cofactor for pyruvate dehydrogenase; sudden glucose availability forces high pyruvate flux, and without thiamine, pyruvate accumulates and is shunted to lactate, causing lactic acidosis

Thiamine (vitamin B1) is an essential cofactor for pyruvate dehydrogenase (PDH), alpha-ketoglutarate dehydrogenase, and transketolase. During refeeding, the rapid influx of glucose drives glycolysis, producing large amounts of pyruvate. PDH normally converts pyruvate to acetyl-CoA for the TCA cycle, but without thiamine, PDH is nonfunctional. Pyruvate accumulates and is reduced to lactate by LDH, causing severe lactic acidosis. This is the mechanism behind Wernicke's encephalopathy and cardiac failure in refeeding — not oxidative stress, lipogenesis failure, or insulin signaling impairment.

A 45-year-old patient with Crohn's disease and prior ileal resection presents with megaloblastic anemia unresponsive to oral folate supplementation. The MOST likely mechanism explaining why folate supplementation alone fails to resolve the anemia is:

Answer: Folate in the methyl-trap: without cobalamin-dependent methionine synthase activity, 5-methylTHF cannot be converted back to THF, depleting the folate pool available for thymidylate synthesis

Following ileal resection, B12 absorption is impaired because intrinsic factor-B12 complexes are absorbed exclusively in the terminal ileum. Without B12, methionine synthase cannot convert 5-methylTHF to THF. This traps all folate in the 5-methylTHF form (the methyl trap), preventing it from being recycled into the active THF pool needed for thymidylate and purine synthesis. Providing more folate cannot bypass this trap — it simply adds more substrate that becomes irreversibly methylated. Parenteral B12 corrects the functional folate deficiency. Folate absorption occurs primarily in the jejunum, not the ileum.

A clinical nutritionist is optimizing protein delivery for a mechanically ventilated patient with acute kidney injury (AKI) NOT on renal replacement therapy (RRT), with a measured GFR of 18 mL/min. Current ASPEN/SCCM guidelines for this specific scenario recommend:

Answer: Moderate protein delivery of 1.2–1.5 g/kg/day, with reassessment if RRT is initiated, at which point protein should increase to 1.5–2.5 g/kg/day

ASPEN/SCCM guidelines distinguish AKI-without-RRT from AKI-on-RRT. For non-RRT AKI in the ICU, protein should not be severely restricted (restriction does not meaningfully delay RRT initiation and worsens catabolism), but full high-protein delivery used for septic or other ICU patients is also not recommended due to uremic solute accumulation. The evidence-supported approach is moderate protein (≈1.2–1.5 g/kg/day) with upward titration to 1.5–2.5 g/kg once continuous RRT begins, since dialysis itself removes amino acids and increases protein requirements. Protein-free nutrition is never guideline-recommended in ICU patients with AKI.