HDL mediates reverse cholesterol transport, removing cholesterol from peripheral tissues and arterial walls via ABCA1 and SR-BI pathways, then transferring it to the liver via CETP-mediated exchange with apoB-containing lipoproteins. Higher HDL is associated with reduced cardiovascular risk.

Gaucher disease is caused by deficiency of glucocerebrosidase (beta-glucosidase), leading to accumulation of glucocerebroside in macrophages of the reticuloendothelial system. Type 1 is the non-neuronopathic form with hepatosplenomegaly; Type 2 and 3 involve CNS involvement.

Dietary cholesterol intake inversely affects endogenous cholesterol synthesis through feedback inhibition of HMG-CoA reductase. However, saturated and trans fats have a greater impact on LDL cholesterol than dietary cholesterol itself. Genetic factors are the primary determinant of serum cholesterol.

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.

Cholesterol is the universal precursor for steroid hormone synthesis (cortisol, testosterone, estrogen) and bile acid synthesis. The side-chain cleavage enzyme (P450scc) catalyzes the first committed step of hormone synthesis, while 7-alpha-hydroxylase initiates bile acid synthesis.

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.

Sterol 7-alpha-hydroxylase catalyzes the first committed step of bile acid synthesis, forming 7-alpha-hydroxycholesterol from cholesterol. This is the rate-limiting step and is regulated by feedback inhibition by bile acids and activation by cholesterol.

Unsaturated fatty acids with cis double bonds introduce kinks in the carbon chain, preventing tight packing and increasing membrane fluidity. Cholesterol and saturated fatty acids decrease fluidity. Temperature and cholesterol content can be modulated to maintain optimal fluidity.

Severe hypertriglyceridemia with eruptive xanthomas and lipemia retinalis indicates predominantly elevated chylomicrons and/or VLDL. This is seen in Type I (lipoprotein lipase deficiency), Type IV (VLDL overproduction), or Type V hyperlipoproteinemia.

VLDL triglyceride content increases during increased hepatic triglyceride synthesis (fed state, fatty liver). HDL is the smallest and most dense; chylomicrons have the lowest protein percentage; LDL particle size varies (pattern A = large, pattern B = small and dense).