C. Methemoglobinemia is the correct answer as it is caused by the oxidation of the ferrous iron to the ferric state in the heme prosthetic group of hemoglobin. This condition can be induced by certain drugs such as nitrates and is characterized by chocolate cyanosis, where the skin and mucous membranes appear blue, and the blood appears chocolate-colored due to the presence of dark-colored methemoglobin. Symptoms include anxiety, headache, dyspnea, and in rare cases, coma and death. Acquired methemoglobinemia can be caused by benzocaine, an aromatic amine used as a topical anesthetic.

Hemoglobin C disease is a nonsickling disease because in this condition, the polar glutamate is replaced by polar lysine, rather than the nonpolar valine found in sickle cell anemia.

Individuals with both HbS (sickle cell hemoglobin) and hereditary persistence of HbF (fetal hemoglobin) would have a decreased extent of red blood cell sickling. This is because HbF reduces the concentration of HbS and inhibits the polymerization of deoxygenated HbS.

Collagen and elastin are fibrous proteins that play structural roles in the body's extracellular matrix (ECM). Collagen and elastin are present in various tissues, including skin, connective tissue, blood vessel walls, and the sclera and cornea of the eye. These fibrous proteins have unique mechanical properties due to their specific amino acid composition and regular secondary structural elements. In contrast to globular proteins, which have complex interactions between secondary, tertiary, and sometimes quaternary structural elements, fibrous proteins like collagen and elastin have a distinct structure.

Collagen is the most abundant protein in the human body. It consists of three polypeptide α chains wound around each other in a triple helix structure. The specific types and organization of collagen vary depending on their structural role in different organs. Collagen may be dispersed as a gel in some tissues, providing support in the ECM or vitreous humor of the eye. In other tissues, collagen is bundled tightly in parallel fibers, providing strength in structures like tendons. In the cornea, collagen is stacked to minimize light scattering. Collagen in bone is arranged as fibers at an angle to resist mechanical shear from any direction.

The collagen superfamily includes more than 25 collagen types and additional proteins with collagen-like domains. The α chains of collagen are held together by interchain hydrogen bonds. Variations in the amino acid sequence of the α chains result in different structural components with similar sizes but slightly different properties. Different combinations of α chains form the various types of collagen found in tissues. For example, type I collagen contains two α1 chains and one α2 chain (α12α2), while type II collagen consists of three α1 chains (α13). Collagens can be categorized into three groups based on their location and functions in the body.
B. Hemoglobin SC disease
C. Methemoglobinemia
D. Sickle cell anemia
E. β-Thalassemia
Correct answer = C. Oxidation of the ferrous (Fe2+) iron to the ferric (Fe3+) state in the heme prosthetic group of hemoglobin forms methemoglobin. This may be caused by the action of certain drugs such as nitrates. The methemoglobinemias are characterized by chocolate cyanosis (a blue coloration of the skin and mucous membranes and chocolate-colored blood) as a result of the dark-colored methemoglobin. Symptoms are related to tissue hypoxia and include anxiety, headache, and dyspnea. In rare cases, coma and death can occur. [Note: Benzocaine, an aromatic amine used as a topical anesthetic, is a cause of acquired methemoglobinemia.] .6. Why is hemoglobin C disease a nonsickling disease?
In HbC, the polar glutamate is replaced by polar lysine rather than by nonpolar valine as in HbS.
.7. What would be true about the extent of red blood cell sickling in individuals with HbS and hereditary persistence of HbF?
It would be decreased because HbF reduces HbS concentration. It also inhibits polymerization of deoxy HbS.
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I. OVERVIEW
Collagen and elastin are examples of common, well-characterized fibrous proteins of the extracellular matrix (ECM) that serve structural functions in the body. For example, collagen and elastin are found as components of skin, connective tissue, blood vessel walls, and the sclera and cornea of the eye. Each fibrous protein exhibits special mechanical properties, resulting from its unique structure, which is obtained by combining specific amino acids into regular, secondary structural elements. This is in contrast to globular proteins (discussed in Chapter 3), whose shapes are the result of complex interactions between secondary, tertiary, and, sometimes, quaternary structural elements.
II. COLLAGEN
Collagen is the most abundant protein in the human body. A typical collagen molecule is a long, rigid structure in which three polypeptides (referred to as α chains) are wound around one another in a rope-like triple helix (Fig. 4.1). Although these molecules are found throughout the body, their types and organization are dictated by the structural role collagen plays in a particular organ. In some tissues, collagen may be dispersed as a gel that gives support to the structure, as in the ECM or the vitreous humor of the eye. In other tissues, collagen may be bundled in tight, parallel fibers that provide great strength, as in tendons. In the cornea of the eye, collagen is stacked so as to transmit light with a minimum of scattering. Collagen of bone occurs as fibers arranged at an angle to each other so as to resist mechanical shear from any direction.
A. Types
The collagen superfamily of proteins includes >25 collagen types as well as additional proteins that have collagen-like domains. The three polypeptide α chains are held together by interchain hydrogen bonds. Variations in the amino acid sequence of the α chains result in structural components that are about the same size (~1,000 amino acids long) but with slightly different properties. These α chains are combined to form the various types of collagen found in the tissues. For example, the most common collagen, type I, contains two chains called α1 and one chain called α2 (α12α2), whereas type II collagen contains three α1 chains (α13). The collagens can be organized into three groups, based on their location and functions in the body (Fig. 4.2).
1.