Dim-vision in the evening and night results from the deficiency of which one of the following ?

1. Basics of Human Nutrition: Macronutrients and Micronutrients (basic)

To understand human health, we must first look at the fuel that drives us. Nutrition is the process by which complex food components are broken down into simpler forms to be utilized by the body for growth, repair, and energy Science-Class VII, Life Processes in Animals, p.122. Our diet is essentially a mixture of various ingredients—like the grains, pulses, and vegetables we see in a sprout salad—each providing different chemical components called nutrients Science, Class VIII, Nature of Matter: Elements, Compounds, and Mixtures, p.117.

These nutrients are broadly classified into two categories based on the quantity our body requires: Macronutrients and Micronutrients. Macronutrients include carbohydrates, proteins, and fats. These are our "staple" requirements, often derived from grains like rice, wheat, and millets, providing the bulk of our daily energy and structural building blocks Exploring Society: India and Beyond, Unity in Diversity, or 'Many in the One', p.128.

Micronutrients, which consist of vitamins and minerals, are needed in much smaller quantities—sometimes only in milligrams or micrograms. Despite their small concentration, they are vital for biochemical reactions. Key minerals include Iron (Fe), Zinc (Zn), and Manganese (Mn) Environment, Shankar IAS Academy, Agriculture, p.363. A deficiency in these can lead to serious health issues, a condition often termed "hidden hunger" because the person may be getting enough calories (macronutrients) but lacks the essential elements for the body to function correctly Economics, Class IX, Food Security in India, p.54.

Feature	Macronutrients	Micronutrients
Required Quantity	Large amounts (grams)	Small amounts (mg/μg)
Main Function	Energy production and tissue growth	Regulating metabolism and protecting health
Examples	Carbohydrates, Proteins, Fats	Vitamins (A, B, C, etc.) and Minerals (Iron, Iodine)

Sources: Science-Class VII, Life Processes in Animals, p.122; Science, Class VIII, Nature of Matter: Elements, Compounds, and Mixtures, p.117; Exploring Society: India and Beyond, Unity in Diversity, or 'Many in the One', p.128; Environment, Shankar IAS Academy, Agriculture, p.363; Economics, Class IX, Food Security in India, p.54

2. Classification of Vitamins: Fat-Soluble vs. Water-Soluble (basic)

Vitamins are essential organic micronutrients that our bodies need in small quantities to maintain life-sustaining processes. Because the human body cannot synthesize most of these vitamins on its own, they must be obtained through our diet Science-Class VII, Adolescence: A Stage of Growth and Change, p.80. The most fundamental way to classify vitamins is based on their solubility—whether they dissolve in water or in fats. This distinction is crucial because it determines how a vitamin is absorbed, transported, stored, and eventually excreted from your body.

Water-soluble vitamins (the B-complex group and Vitamin C) dissolve easily in water. Because our blood is water-based, these vitamins move freely through the bloodstream. However, they are generally not stored in large amounts; any excess is filtered by the kidneys and excreted in urine. This means we need a regular, daily supply of these vitamins. In contrast, fat-soluble vitamins (A, D, E, and K) require lipids (fats) and bile for absorption in the small intestine Science-Class VII, Life Processes in Animals, p.134. Once absorbed, they are stored in the liver and fatty (adipose) tissues for long periods.

This storage capability of fat-soluble vitamins is a double-edged sword. While it allows us to go for periods without consuming them, it also means that taking them in excessive amounts can lead to toxic accumulation, a condition known as hypervitaminosis. Interestingly, the property of being soluble in fats is also what allows certain environmental pollutants to persist and concentrate in the bodies of organisms as they move up the food chain, a process known as biomagnification Environment, Shankar IAS Academy, Functions of an Ecosystem, p.16.

Feature	Water-Soluble Vitamins	Fat-Soluble Vitamins
Vitamins Included	B-complex (B₁, B₂, B₁₂, etc.) and Vitamin C	A, D, E, and K
Absorption	Directly into the blood	First into the lymph, then the blood; requires fats
Storage	Not stored (excreted in urine)	Stored in liver and fatty tissues
Toxicity	Rare (excess is peed out)	Possible if taken in extreme excess

Sources: Science-Class VII, Adolescence: A Stage of Growth and Change, p.80; Science-Class VII, Life Processes in Animals, p.134; Environment, Shankar IAS Academy, Functions of an Ecosystem, p.16

3. Major Nutritional Deficiency Diseases (intermediate)

In our journey through human nutrition, we must understand that our bodies are sophisticated biochemical laboratories. While macronutrients (carbohydrates, fats, proteins) provide the energy and building blocks, micronutrients—vitamins and minerals—act as the essential catalysts for life-sustaining reactions. When these are missing, the body's internal machinery begins to stall, leading to Nutritional Deficiency Diseases. These are often categorized as "hidden hunger" because a person might consume enough calories but still suffer from a lack of vital nutrients.

One of the most critical deficiencies involves Vitamin A (Retinol). To understand why its absence causes vision issues, we look at the retina of the eye, which contains two types of photoreceptor cells: cones (for color and bright light) and rods (for low-light vision). Vitamin A is a precursor to rhodopsin, a pigment found in the rods that is extremely sensitive to light. When Vitamin A levels drop, the body cannot regenerate rhodopsin quickly enough, leading to Nyctalopia (night blindness). If left untreated, this can progress to xerophthalmia, where the eyes become dangerously dry, eventually leading to permanent blindness Science-Class VII, Adolescence: A Stage of Growth and Change, p. 80.

Another major concern, particularly in the Indian context, is Anaemia. This condition arises from a deficiency of Iron or Vitamin B₁₂. Iron is the central component of hemoglobin, the protein in red blood cells that carries oxygen from the lungs to the rest of the body. Without enough iron, the body cannot produce sufficient hemoglobin, leading to fatigue and weakness. Adolescents, especially girls, are at a higher risk for these blood-related health issues due to rapid growth and physiological changes Science-Class VII, Adolescence: A Stage of Growth and Change, p. 80. Other classic deficiencies include Scurvy (Vitamin C), which affects collagen synthesis, and Goitre (Iodine), which impairs the thyroid gland Science, Class VIII, Health: The Ultimate Treasure, p. 35.

Nutrient	Deficiency Disease	Primary Symptom
Vitamin A	Night Blindness / Xerophthalmia	Inability to see in dim light
Iron / B₁₂	Anaemia	Weakness, pale skin, fatigue
Vitamin C	Scurvy	Bleeding gums, slow wound healing
Iodine	Goitre	Swollen thyroid gland in the neck

Sources: Science-Class VII, Adolescence: A Stage of Growth and Change, p.80; Science, Class VIII, Health: The Ultimate Treasure, p.35

4. Public Health Initiatives: Food Fortification and Nutrition Missions (intermediate)

To understand public health in India, we must look at two pillars: Regulation (ensuring food is safe) and Intervention (ensuring food is available and nutritious). At the heart of regulation is the Food Safety and Standards Authority of India (FSSAI). Established under the Food Safety and Standards Act, 2006, FSSAI serves as an autonomous body under the Ministry of Health & Family Welfare. Its primary role is to set science-based standards for food articles and to regulate their manufacture, storage, and distribution to ensure safe consumption Indian Economy, Nitin Singhania (ed 2nd 2021-22), Food Processing Industry in India, p.411. Before this Act, food regulation was scattered across various departments; the 2006 Act created a 'single line of command' for food safety across the country Indian Economy, Vivek Singh (7th ed. 2023-24), Supply Chain and Food Processing Industry, p.374.
One of the most effective public health strategies managed by the government is Food Fortification. This involves deliberately increasing the content of essential micronutrients (like Iron, Vitamin A, or Iodine) in staple foods to improve their nutritional quality. When you see the FSSAI symbol or the +F logo on food packets, it indicates that the product has met government safety and quality standards Exploring Society: India and Beyond, Social Science-Class VII, Understanding Markets, p.269. This is particularly vital for combating 'hidden hunger' — a condition where individuals may get enough calories but lack the vitamins and minerals necessary for growth and disease prevention.
On the intervention side, the Pradhan Mantri Poshan Shakti Abhiyan (PM-POSHAN), formerly known as the Mid-Day Meal Scheme, is a flagship nutrition mission. It targets students from Classes I to VIII in government and government-aided schools. This scheme is unique because it addresses two critical development goals simultaneously: it improves the nutritional intake (specifically calories and protein) of children and incentivizes school enrollment while reducing dropout rates Economics, Class IX, Poverty as a Challenge, p.39.

Sources: Indian Economy, Nitin Singhania (ed 2nd 2021-22), Food Processing Industry in India, p.411; Indian Economy, Vivek Singh (7th ed. 2023-24), Supply Chain and Food Processing Industry, p.374; Exploring Society: India and Beyond, Social Science-Class VII, Understanding Markets, p.269; Economics, Class IX, Poverty as a Challenge, p.39

5. Anatomy of the Eye: Photoreceptors (Rods and Cones) (exam-level)

The human retina is a biological masterpiece—a thin, delicate membrane where light is converted into neural signals that the brain can interpret as images Science Class X, The Human Eye and the Colourful World, p.162. To perform this conversion, the retina relies on millions of specialized light-sensitive nerve cells called photoreceptors. These cells are a classic example of how cellular structure is precisely adapted to its function; their elongated shapes allow them to maximize light absorption Science Class VIII, The Invisible Living World: Beyond Our Naked Eye, p.13. We categorize these photoreceptors into two primary types: Rods and Cones.

Rods are incredibly sensitive to light intensity, making them responsible for our vision in twilight or dim conditions (scotopic vision). However, they do not perceive color and provide lower visual acuity. Their secret lies in a light-sensitive pigment called Rhodopsin (or visual purple). Conversely, Cones are responsible for high-acuity daytime vision and color perception (photopic vision). They only function in bright light and contain pigments that respond to specific wavelengths (red, green, and blue).

Feature	Rods	Cones
Vision Type	Dim light / Night vision	Bright light / Day vision
Color Perception	None (Grayscale)	High (RGB perception)
Key Pigment	Rhodopsin	Photopsin / Iodopsin
Sensitivity	Very High (can detect single photons)	Lower (requires more light)

From a nutritional perspective, the health of rod cells is directly tethered to Vitamin A (Retinol). Vitamin A is the essential chemical precursor for synthesizing retinal, which combines with a protein called opsin to form Rhodopsin. When Vitamin A levels are low, the body cannot adequately regenerate Rhodopsin after it is bleached by light. This biochemical failure manifests as Nyctalopia (night blindness), which is the earliest clinical sign of a progressive nutritional deficiency known as Xerophthalmia Science Class VII, Adolescence: A Stage of Growth and Change, p.80.

Sources: Science Class X, The Human Eye and the Colourful World, p.162; Science Class VIII, The Invisible Living World: Beyond Our Naked Eye, p.13; Science Class VII, Adolescence: A Stage of Growth and Change, p.80

6. Vitamin A (Retinol) and the Visual Cycle (exam-level)

To master the concept of vision, we must first look at the retina, the delicate membrane at the back of our eye that acts like a biological screen. As we learn in Science, Class X, The Human Eye and the Colourful World, p.162, the retina contains an enormous number of light-sensitive cells that convert light into electrical signals for the brain. These cells come in two distinct varieties, each requiring specific nutrients to function.

Vitamin A (Retinol) is the fundamental building block for a photopigment called rhodopsin (also known as visual purple). Rhodopsin is found exclusively in the rod cells of the retina. When even a tiny amount of light enters the eye, it strikes the rhodopsin, causing a chemical change that triggers a nerve impulse. This allows us to see in low-light or dim conditions. Without sufficient Vitamin A, the body cannot regenerate rhodopsin quickly enough, leading to a condition called nyctalopia or night blindness.

Feature	Rod Cells	Cone Cells
Primary Function	Vision in dim light (scotopic vision)	Color vision and detail in bright light (photopic vision)
Key Pigment	Rhodopsin (Vitamin A dependent)	Iodopsin
Deficiency Impact	Night Blindness	Color blindness (usually genetic, not nutritional)

If Vitamin A deficiency persists, it progresses beyond night blindness to a more severe condition known as xerophthalmia Science-Class VII, Adolescence: A Stage of Growth and Change, p.80. This begins with the drying of the conjunctiva and can lead to the softening of the cornea (keratomalacia), eventually causing permanent blindness. This is why a diet rich in precursors like beta-carotene (found in carrots and leafy greens) or preformed Vitamin A (found in animal products) is essential for maintaining the structural and functional integrity of the visual cycle.

Sources: Science, Class X, The Human Eye and the Colourful World, p.162; Science-Class VII, Adolescence: A Stage of Growth and Change, p.80

7. Solving the Original PYQ (exam-level)

Now that you have mastered the roles of different micronutrients, you can see how these building blocks come together in this question. The critical link here is the relationship between Vitamin A (Retinol) and the visual cycle. As we learned, Vitamin A is the essential precursor for the synthesis of rhodopsin, a light-sensitive pigment found in the rod cells of the retina. Because rods are the primary cells responsible for scotopic vision (vision in dim light), a lack of Vitamin A prevents the eye from adapting to low-light environments, leading to night blindness. This fundamental physiological connection is a cornerstone of nutritional science as outlined in Science-Class VII . NCERT(Revised ed 2025).

To solve this like a seasoned aspirant, focus on the keyword "dim-vision." When you see this, your mind should immediately pivot to the health of the retina's rod cells. The correct answer is (A) Vitamin A because it is the only nutrient listed that serves as a direct chemical component of visual pigments. Identifying this symptom as the earliest clinical sign of xerophthalmia allows you to bridge the gap between a simple deficiency and a progressive disease, a common theme in UPSC biology questions.

UPSC often uses other essential vitamins as traps to test the precision of your knowledge. While Vitamin C is vital, its deficiency leads to scurvy (connective tissue issues), not vision loss. Vitamin B12 is crucial for nerve health and red blood cell formation, and Vitamin E serves as a powerful antioxidant; however, neither is involved in the photochemical reactions required for night vision. By distinguishing between these specific metabolic roles, you can avoid common pitfalls and confidently select the correct physiological pathway.