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

A critically ill patient on total parenteral nutrition develops refeeding syndrome. Which electrolyte abnormality most directly drives the life-threatening cardiac arrhythmias seen in this condition?

Answer: Hypophosphatemia

Refeeding syndrome is characterized by a rapid intracellular shift of phosphate driven by insulin release following carbohydrate administration. Severe hypophosphatemia impairs ATP synthesis, causing myocardial dysfunction and potentially fatal arrhythmias. While hypomagnesemia also occurs and contributes, hypophosphatemia is the primary electrolyte disturbance underlying the cardiac manifestations.

In a patient with short bowel syndrome retaining only 80 cm of jejunum with an intact ileocecal valve and colon, which adaptation mechanism contributes MOST to long-term enteral autonomy?

Answer: Colonic fermentation of unabsorbed carbohydrates to short-chain fatty acids

With an intact colon, colonic bacteria ferment unabsorbed complex carbohydrates into short-chain fatty acids (butyrate, propionate, acetate), which can provide up to 500–1000 kcal/day and are readily absorbed by colonocytes. This colonic energy salvage pathway is the dominant adaptation that enables enteral autonomy in patients retaining their colon, making dietary optimization toward complex carbohydrates a key therapeutic strategy.

A 58-year-old patient with alcoholic cirrhosis is started on a high-protein diet to address sarcopenia. Which metabolic derangement MOST limits the use of standard branched-chain amino acid catabolism in this patient's skeletal muscle?

Answer: Decreased activity of branched-chain α-keto acid dehydrogenase complex in muscle

In cirrhosis, skeletal muscle becomes a primary site of ammonia detoxification via glutamine synthesis, consuming α-ketoglutarate. The resulting depletion of TCA cycle intermediates downregulates branched-chain α-keto acid dehydrogenase (BCKDH) complex activity, impairing complete oxidation of BCAAs in muscle. This creates a metabolic block specifically in BCAA catabolism at the mitochondrial level, which is distinct from — though compounded by — impaired hepatic urea synthesis.

Which statement BEST describes the metabolic consequence of providing greater than 50% of non-protein calories as lipid in a mechanically ventilated ARDS patient on parenteral nutrition?

Answer: Reduced CO₂ production lowers the respiratory quotient, potentially decreasing ventilatory demand

Fat has a respiratory quotient (RQ) of approximately 0.7 versus 1.0 for glucose. Providing the majority of non-protein calories as lipid lowers the overall RQ, reducing CO₂ production per unit of energy delivered. This is clinically relevant in ventilator-dependent ARDS patients because decreased CO₂ load can reduce the required minute ventilation, potentially facilitating weaning. While soy-based emulsion pro-inflammatory effects are a real concern, the primary clinical rationale for higher lipid fractions in this context is the favorable CO₂ profile.

A patient with carcinoid syndrome develops pellagra-like dermatitis, diarrhea, and cognitive changes. What is the precise metabolic mechanism responsible?

Answer: Tumor sequestration of tryptophan for serotonin synthesis depletes the precursor pool for NAD⁺ biosynthesis

In carcinoid syndrome, tumor cells consume large amounts of tryptophan to produce serotonin. Normally, up to 60 mg of dietary tryptophan is converted to 1 mg of niacin via the kynurenine pathway, making tryptophan a key endogenous source of NAD⁺ precursors. Massive serotonin synthesis by the tumor exhausts available tryptophan, leaving inadequate substrate for the kynurenine pathway and NAD⁺ production, resulting in functional niacin (vitamin B3) deficiency — identical in presentation to classic pellagra.

During prolonged starvation (>5 days), the brain reduces its glucose dependency by utilizing ketone bodies. Which enzymatic step is rate-limiting for ketone body UTILIZATION in neurons, and why cannot the liver use this same pathway?

Answer: Succinyl-CoA:3-ketoacid CoA transferase (SCOT), which is absent in hepatocytes

Ketone body utilization requires succinyl-CoA:3-ketoacid CoA transferase (SCOT, also called 3-oxoacid CoA-transferase) to transfer CoA from succinyl-CoA to acetoacetate, forming acetoacetyl-CoA for further oxidation. Hepatocytes constitutively lack SCOT expression — this is not a regulatory downregulation but a permanent absence — which prevents the liver from oxidizing the ketone bodies it produces. This tissue-specific enzyme absence ensures hepatically-synthesized ketones are obligately exported as fuel for extrahepatic organs including the brain.