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Macrocytic Anemia

Malnutrition

Maple syrup urine disease (MSUD)

Maple Syrup Urine Disease (MSUD): Causes, Symptoms, Diagnosis, and Treatment Maple Syrup Urine Disease (MSUD) is a rare genetic metabolic disorder that affects the body's ability to break down certain amino acids: leucine, isoleucine, and valine. This leads to a toxic buildup of these amino acids and their byproducts in the blood, which can cause severe neurological damage and life-threatening complications if left untreated. The condition gets its name from the distinct sweet, maple syrup-like odor in the urine and sweat of affected individuals. Causes and Genetics MSUD is an autosomal recessive disorder, meaning a child must inherit two defective copies of the BCKDHA, BCKDHB, or DBT gene—one from each parent. These genes encode for the branched-chain ketoacid dehydrogenase (BCKD) complex, which is responsible for breaking down branched-chain amino acids (BCAAs). When this enzyme complex is deficient or malfunctioning, these amino acids accumulate to toxic levels.

Mast Cell Activation Syndrome (MCAS)

Key Takeaways Mast Cell Activation Syndrome (MCAS) is a condition in which mast cells release excessive inflammatory chemicals, such as histamine, tryptase, prostaglandins, and leukotrienes. These mediators can cause symptoms affecting multiple body systems, including the skin, digestive tract, respiratory system, cardiovascular system, and nervous system. Common symptoms include flushing, hives, abdominal pain, diarrhea, wheezing, rapid heart rate, fatigue, and brain fog. Diagnosis typically requires three components: compatible symptoms in multiple organ systems, laboratory evidence of mast cell mediator release, and improvement with mast cell–targeted treatment. Treatment focuses on trigger avoidance, antihistamines, mast cell stabilizers, and other medications that reduce mediator release or block their effects. What Is Mast Cell Activation Syndrome? Mast Cell Activation Syndrome (MCAS) is a disorder in which mast cells become overactive and release inflammatory chemicals too frequently or in excessive amounts.

Mastocytosis

Megaloblastic Anemia

Meningitis

Menopause

Menopause is the permanent cessation of menstrual periods, defined as 12 consecutive months without a period in the absence of other causes. It marks the end of a woman's reproductive years and is a natural biological transition, not a disease — but the hormonal changes it brings have far-reaching effects on nearly every organ system, and the years surrounding it are associated with significant health risks that deserve proactive attention. Menopause occurs because the ovaries exhaust their supply of follicles — the structures that produce eggs and hormones. As follicle reserves deplete, estradiol production falls and follicle-stimulating hormone (FSH) rises. The average age of natural menopause in Western populations is 51, with most women transitioning between ages 45 and 55. Menopause occurring before age 40 is classified as premature ovarian insufficiency (POI) and carries specific clinical implications.

Menstrual Irregularity

Metabolic Acidosis

Metabolic acidosis is a common acid–base disorder that occurs when there is either too much acid in the body or too little bicarbonate, causing blood pH to fall. It is often first detected on routine blood tests showing low bicarbonate and changes in the anion gap, sometimes before symptoms become severe. This condition develops when acid production increases, acid excretion by the kidneys is impaired, or bicarbonate is lost from the body. The result is a disruption in the body’s normal acid–base balance, which is essential for proper cellular function. Types and Mechanisms Metabolic acidosis is typically classified based on the anion gap, a key lab calculation used to identify the underlying cause: High anion gap metabolic acidosis: Caused by the accumulation of unmeasured acids in the blood.

Metabolic alkalosis

Metabolic disorders

Metabolic syndrome

Metabolic Syndrome: Causes, Risk Factors, Diagnosis, and Management What is Metabolic Syndrome? Metabolic syndrome is a cluster of conditions that together increase the risk of cardiovascular disease, type 2 diabetes, and other serious health complications. It is characterized by a combination of high blood pressure, elevated blood sugar levels, excess abdominal fat, and abnormal cholesterol or triglyceride levels. The presence of three or more of these factors is typically used to diagnose metabolic syndrome. Causes and Risk Factors Metabolic syndrome is primarily driven by insulin resistance, a condition in which the body's cells do not respond effectively to insulin. This leads to elevated blood sugar levels and a cascade of metabolic imbalances. Key risk factors include: Obesity – Particularly central (abdominal) obesity, often measured by waist circumference. Sedentary Lifestyle – Lack of physical activity contributes to insulin resistance and weight gain.

Methylmalonic acidemia (MMA)

Methylmalonic Acidemia (MMA): A Comprehensive Guide What is Methylmalonic Acidemia (MMA)? Methylmalonic acidemia (MMA) is a rare genetic metabolic disorder that impairs the body’s ability to break down certain proteins and fats. This condition is caused by mutations in genes responsible for processing methylmalonyl-CoA, leading to the accumulation of toxic substances, including methylmalonic acid, in the blood and tissues. MMA can result in serious health issues, especially if not diagnosed and managed early. It is one of the most well-known organic acidemias, a group of disorders that affect how the body processes specific amino acids and lipids. What Causes MMA? MMA is caused by genetic mutations in the following pathways: Deficiency of Methylmalonyl-CoA Mutase (MUT): The enzyme responsible for converting methylmalonyl-CoA into succinyl-CoA is defective or absent. Vitamin B12 (Cobalamin) Processing Defects: MMA can also result from errors in the body’s ability to process vitamin B12, which is essential for methylmalonyl-CoA mutase function.

Microcytic Anemia

Mild Cognitive Impairment (MCI)

Mixed connective tissue disease (MCTD)

Mixed Connective Tissue Disease (MCTD) is an autoimmune disorder characterized by features commonly seen in three other connective tissue diseases: systemic lupus erythematosus, scleroderma, and polymyositis. MCTD is thus considered an overlap syndrome. Patients with MCTD may exhibit a variety of symptoms, including joint pain, muscle weakness, Raynaud's phenomenon (a condition where fingers and toes turn white or blue and feel cold in response to cold temperatures or stress), and problems with internal organs such as the lungs and kidneys. A distinctive marker for MCTD is the presence of high levels of anti-U1 ribonucleoprotein (anti-U1 RNP) antibodies in the blood. The course of MCTD varies among individuals; some experience only mild symptoms, while others may have a more severe progression requiring active management. Treatment is tailored to the individual's symptoms and may include medications that suppress the immune system, such as corticosteroids and immunosuppressants.

Mold Allergy

Mold exposure

Monoclonal Gammopathy of Undetermined Significance (MGUS)

What Is MGUS? Monoclonal Gammopathy of Undetermined Significance (MGUS) is a benign condition in which an abnormal protein—called a monoclonal protein or M protein—is found in the blood. Key facts about MGUS: Not cancer — it is a benign condition Affects 3–4% of adults over age 50 Usually causes no symptoms Requires monitoring but often no treatment Risk of progression to cancer is low (~1% per year) How MGUS Is Diagnosed MGUS is often discovered during routine blood tests or while evaluating other conditions. Diagnosis typically includes: Protein electrophoresis (SPEP): Detects abnormal protein bands Immunofixation: Identifies the specific type of protein M protein level: Usually less than 3 g/dL in MGUS Bone marrow evaluation: Less than 10% plasma cells No organ damage: No bone lesions, kidney dysfunction, anemia, or elevated calcium MGUS vs.

Multiple Endocrine Neoplasia type 2 (MEN2)

Multiple Myeloma

Multiple Myeloma is a complex and aggressive hematological malignancy characterized by the uncontrolled proliferation of plasma cells in the bone marrow. Plasma cells, a type of white blood cell, are crucial for producing antibodies (immunoglobulins) that fight infection. In Multiple Myeloma, these cells become cancerous (myeloma cells) and produce large amounts of a single type of abnormal antibody known as a monoclonal protein, or M protein, which can be detected in blood or urine. These malignant plasma cells crowd out healthy blood cells and produce substances that cause bone destruction, leading to pain and fractures. The pathophysiology of Multiple Myeloma involves genetic and epigenetic alterations, leading to the disruption of normal plasma cell growth and death. Common genetic aberrations include chromosomal translocations and deletions, particularly involving chromosome 14. These changes result in the overexpression of oncogenes or the loss of tumor suppressor genes, promoting myeloma cell survival and proliferation.

Multiple sclerosis (MS)

Multiple Sclerosis (MS) is a chronic and often debilitating neurological condition that affects the central nervous system, which includes the brain and spinal cord. This autoimmune disease occurs when the body's immune system mistakenly attacks the protective covering of nerve fibers, known as myelin, leading to inflammation and damage. This damage disrupts the communication between the brain and the rest of the body, resulting in a wide range of symptoms that vary from person to person. Common symptoms of MS include fatigue, numbness or tingling in the limbs, muscle weakness, vision problems, and difficulties with coordination and balance. Over time, these symptoms can become more severe, leading to mobility issues and other significant health challenges. While the exact cause of MS is unknown, it is believed to involve a combination of genetic and environmental factors. There is currently no cure for multiple sclerosis, but there are various treatment options available that can help manage symptoms, reduce the frequency of relapses, and slow the progression of the disease.

Muscle Trauma

Muscular Dystrophy

Myasthenia Gravis

What Is Myasthenia Gravis? Myasthenia gravis (MG) is a chronic autoimmune neuromuscular disorder that causes weakness in the skeletal muscles—the muscles your body uses for movement. The name comes from Latin and Greek, meaning “grave muscle weakness,” though with proper treatment, many people with MG lead normal or near-normal lives. In MG, the immune system mistakenly produces antibodies that interfere with the communication between nerves and muscles, leading to muscle fatigue and weakness that typically worsens with activity and improves with rest. What Causes It? In most cases, MG is caused by antibodies that attack acetylcholine receptors (AChR) at the neuromuscular junction—the place where nerve cells signal muscles to move. Some patients may have antibodies against other proteins involved in this process, such as: MuSK (Muscle-Specific Kinase) LRP4 (Low-Density Lipoprotein Receptor-Related Protein 4) These antibodies block or destroy the receptors needed for muscle contraction, preventing normal muscle activation. Who Can Get Myasthenia Gravis?

Mycoplasma pneumoniae Infection

Mycoplasma pneumoniae is a significant bacterial pathogen known primarily for causing atypical pneumonia, though it can also be associated with a range of other respiratory conditions and extrapulmonary manifestations. Pneumonia is an infection of one or both of the lungs caused by bacteria, viruses, or fungi. Understanding its biology, epidemiology, clinical presentation, diagnostic methods, and treatment is crucial in managing infections caused by this bacterium. Biology and Epidemiology Mycoplasma pneumoniae is unique among bacteria due to its lack of a cell wall, which makes it naturally resistant to antibiotics that target cell wall synthesis, like penicillin. This small bacterium is primarily spread through respiratory droplets, leading to outbreaks in crowded environments such as schools and military barracks. It's more prevalent in late summer and fall, but infections can occur year-round. Clinical Presentation The infection often presents as a mild respiratory illness, but can progress to more severe pneumonia.

Myelodysplastic Syndrome (MDS)

What Is Myelodysplastic Syndrome (MDS)? Myelodysplastic syndrome (MDS) is a group of blood disorders in which the bone marrow does not produce enough healthy, functional blood cells. Instead of maturing normally, blood cells develop abnormally — a process called dysplasia — and many are destroyed before they ever leave the bone marrow. The result is a shortage of one or more types of blood cells: red blood cells, white blood cells, or platelets. MDS is not a single disease but a spectrum of related conditions ranging from low-risk forms that progress slowly over many years to high-risk forms that can transform into acute myeloid leukemia (AML). The course of MDS varies widely — many people have slow-growing forms that can be monitored and managed for years. Understanding where on this spectrum a person's MDS falls is one of the most important parts of diagnosis and treatment planning. MDS is primarily a disease of older adults, with most diagnoses occurring after age 65. It is slightly more common in men than women. In most people, MDS is not passed down in families.

Myeloproliferative Disorders

Myeloproliferative Neoplasms

Myeloproliferative neoplasms (MPNs) are a group of rare blood cancers characterized by the bone marrow's overproduction of one or more types of mature blood cells—red blood cells, white blood cells, or platelets—due to acquired mutations in blood-forming stem cells. The main types of MPNs include polycythemia vera (overproduction of red blood cells), essential thrombocythemia (overproduction of platelets), primary myelofibrosis (bone marrow scarring with abnormal blood cell production), and chronic neutrophilic leukemia (overproduction of neutrophils). Many patients have no symptoms initially and are diagnosed through routine blood tests, but as the disease progresses, symptoms may include fatigue, weakness, headaches, dizziness, night sweats, itching (especially after warm baths), enlarged spleen causing abdominal fullness or pain, bleeding complications, blood clots (which can lead to stroke or heart attack), and bone pain.

Myositis