In the carnitine shuttle system, carnitine palmitoyltransferase I (CPT-I) is inhibited by:
Answer: B
CPT-I is allosterically inhibited by malonyl-CoA, a key regulatory point linking fatty acid synthesis (high during fed state) with fatty acid oxidation (low during fed state).
Q.62Hard
Which of the following conditions would result in elevated levels of lipoprotein(a) (Lp(a))?
Answer: B
Lp(a) levels are primarily determined by genetic factors (LPA gene polymorphisms) and are elevated in chronic kidney disease due to reduced catabolism. Lp(a) is an independent cardiovascular risk factor.
Q.63Hard
A 50-year-old female with hypothyroidism presents with elevated cholesterol. Which mechanism explains the increased cholesterol in hypothyroidism?
Answer: D
Thyroid hormones upregulate LDL receptors and also inhibit HMG-CoA reductase. In hypothyroidism, all these effects are reversed, leading to increased cholesterol levels through multiple mechanisms.
Q.64Hard
The oxidative stress marker oxidized LDL (oxLDL) primarily activates which receptor to promote foam cell formation?
Answer: B
OxLDL is recognized by scavenger receptors (SR-A and LOX-1) on macrophages, not by the classical LDL receptor. This leads to unregulated cholesterol uptake and foam cell formation in atherosclerosis.
Q.65Hard
In Type III hyperlipoproteinemia (Familial Dysbetalipoproteinemia), the primary defect involves:
Answer: A
Type III hyperlipoproteinemia results from apoE2 homozygosity combined with another genetic or environmental factor. ApoE2 has reduced affinity for lipoprotein receptors, impairing remnant particle clearance.
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Q.66Hard
A patient with primary biliary cholangitis shows elevated cholestanol levels. Which enzyme defect is most likely responsible?
Answer: B
Sterol 27-hydroxylase catalyzes the side-chain oxidation of cholesterol to form bile acid precursors. Its deficiency or dysfunction leads to cholestanol accumulation and neuropathy. Elevated cholestanol is a characteristic finding in sterol 27-hydroxylase deficiency.
Q.67Hard
During the fed state, acetyl-CoA carboxylase (ACC) is activated by which mechanism?
Answer: D
In the fed state, high citrate levels allosterically activate ACC, and insulin signaling causes PKB/Akt to phosphorylate and activate ACC. This promotes fatty acid synthesis. Conversely, AMPK phosphorylation inactivates ACC during energy deprivation.
Q.68Hard
A deficiency of apolipoprotein B-48 synthesis would primarily affect which lipid transport pathway?
Answer: B
ApoB-48 is the truncated form of apoB synthesized in intestinal enterocytes and is essential for chylomicron assembly and secretion. Its deficiency causes abetalipoproteinemia, characterized by fat malabsorption and severe deficiency of fat-soluble vitamins.
Q.69Hard
A 35-year-old patient with atherosclerosis has normal LDL cholesterol but elevated lipoprotein(a). Which genetic polymorphism is primarily responsible for high Lp(a) levels?
Answer: B
Lipoprotein(a) levels are predominantly determined by genetic variations in the LPA gene, which encodes apolipoprotein(a). Smaller isoforms are associated with higher plasma Lp(a) levels and increased cardiovascular risk, independent of LDL cholesterol levels.
Q.70Hard
A patient presents with neurodegenerative symptoms, progressive neuropathy, and xanthomas. Plasma cholestanol and cholestane-3-beta,5-alpha,6-beta-triol levels are markedly elevated. Which genetic defect is most likely?
Answer: B
CYP27A1 mutations cause cerebrotendinous xanthomatosis (CTX), characterized by elevated cholestanol and cholestane triol precursors due to impaired side-chain oxidation of cholesterol. This leads to progressive neurological deterioration, cataracts, and diarrhea. Chenodeoxycholic acid therapy is beneficial.
Q.71Hard
Tangier disease is associated with deficiency of which protein?
Answer: B
Tangier disease results from mutations in the ABCA1 gene, which encodes a cholesterol transporter. This leads to severe HDL deficiency and accumulation of cholesterol esters in tissues.
Q.72Hard
In the condition of hypertriglyceridemia with normal LDL levels, which genetic defect is most likely?
Answer: B
Deficiency of lipoprotein lipase or its cofactor apoC-II results in severe hypertriglyceridemia with elevated chylomicrons and VLDL but relatively normal LDL levels.
Q.73Hard
A patient with severely elevated serum triglycerides (>1000 mg/dL) is at immediate risk of:
Answer: B
Severe hypertriglyceridemia (>1000 mg/dL) significantly increases the risk of acute pancreatitis due to pancreatic inflammation caused by triglyceride-rich lipoproteins.
