00 - USMLE - General Principles

What is the function of the Na⁺/K⁺-ATPase?

Uses ATP to move 3 Na⁺ out of the cell and 2 K⁺ into the cell per cycle. Maintains ionic gradients, contributes to resting membrane potential, and supports secondary active transport.

Which pathway generates NADPH for antioxidant defense in red blood cells?

The pentose phosphate pathway, also called the hexose monophosphate shunt. G6PD catalyzes its rate-limiting step.

What is the rate-limiting enzyme of glycolysis?

Phosphofructokinase-1 (PFK-1), which converts fructose-6-phosphate into fructose-1,6-bisphosphate. It is activated by AMP and fructose-2,6-bisphosphate and inhibited by ATP and citrate.

What type of inheritance is typical of Marfan syndrome?

Autosomal dominant. It is commonly caused by a pathogenic variant in FBN1, which encodes fibrillin-1. Clinical associations include aortic root dilation, lens subluxation, and arachnodactyly.

Why does mitochondrial disease often affect muscle and nervous tissue?

These tissues have high energy requirements and depend heavily on ATP production by oxidative phosphorylation. Mitochondrial dysfunction can impair their function, although clinical patterns vary by disorder.