Be parsimonious. My teachers taught me this principle at the bedside in my residency days. As far as possible, I was taught, doctors should try to explain all symptoms and signs by a single diagnosis.

That was in 1980. Those were the days when physicians used their brains and stethoscopes to make a diagnosis. Doctors believed in the virtues of long histories, examined their patients from head to toe, ordered few lab tests, discussed differential diagnosis and took pride in assigning correct explanation for the diseases.

Little did I know then that my teachers were referring to Occam’s razor – a well-known logical principle often applied in medicine. It states that the simplest- and single- explanation for symptoms or signs is usually the correct one. And thus, when patients presented with fever, heart murmur and an enlarged spleen, our diagnosis began, and ended with infective endocarditis. When patients presented with chronic alcohol dependence, swollen legs, yellow eyes, and distended abdomen, we argued that cirrhosis of liver could explain everything.

We seldom thought that nothing prevents a febrile patient with a heart murmur from getting typhoid fever. Or a patient with a 40-pack year long history of smoking often dies of a pulmonary embolism and not because of acute exacerbation of chronic obstructive lung disease. Or an anxious girl presenting with fast respirations in the emergency room could have diabetic keto-acidosis and not a psychosomatic disorder.

Yesterday, a medical student taught me a lesson. Never close your differential diagnosis prematurely. For almost three weeks, this student had stuffy, runny and blocked nose that led to visits to ear, nose and throat specialists. Low-grade fevers, frontal headaches, yellow secretions from nose made a perfect case for acute bacterial sinusitis. She was prescribed oral antibiotics, anti-histamines, pain-killers and medication drops to clear her blocked nose. Her fever went off, only to return a week later. She continued to feel as miserable, and self-diagnosed her sickness to inflamed sinuses and allergies which she seem to be carrying since childhood.

One week had slipped by, but the fever failed to subside. Instead, it began spiking twice a day. She felt nauseous, had difficulty falling and staying asleep and was thoroughly exhausted. Her fear and anxiety was palpable- she was about to take her pre-final exams but was not sure if she was left with enough strength to endure the ordeal.

Her father, a physician, spent 20 hours in a train to be with his daughter. He asked me if I could see her in a guest house where she was temporarily shifted from the hostel. I agreed to make a home call after I finished my office chores.

The medical student looked tired and weak. I asked her few pertinent close-ended questions, probed how it all began and made some mental notes about the intensity, duration and frequency of her febrile episodes. I checked her vitals and examined her sinuses, pharynx and chest. Her nose linings were red and her throat, congested. “My nose is still stuffy and runny,” she said, “and my headaches are really bad”, she complained. “I can’t stand dust- this allergy is too much for me,” she said. I explained to her father that although the treatment was correctly prescribed and taken, it did not work because the bacteria could have been resistant to the antibiotic. I tried to ease the collective anxiety and prescribed a new antibiotic. “These fevers run their own course, but the new antibiotic should work,” I gently assured them.

The father didn’t look convinced but was polite enough to say nothing. I sensed an unvoiced request for a full lab checkup. I asked her to see me in the outpatient department.

Next morning, she arrived in my OPD. I ordered complete blood counts, malaria microscopy, a rapid diagnostic test for malaria, Widal test for typhoid, and radiographs of chest and sinuses. I also sent her for an ENT consultation.

Three hours later, her father called me. She had tested positive for Plasmodium falciparum, a deadly parasite that causes life-threatening malaria.

As I heard the lab result, I felt relieved and ashamed -not necessarily in that order. Relieved, for the lab quickly found an answer to her problem. Ashamed, because the answer didn’t come from my head. The consequences of missed diagnosis and failed treatment began to float in my mind. I explained to her father the need for urgently buying a combination of an oral anti-malarial. Twenty- four hours later, her fevers vanished and didn’t visit her again.

Where did I err? Why malaria didn’t figure in my differential diagnosis? Did I really build a differential diagnosis? I had made up my mind even before she had completed her story. Everything, I felt, fitted so nicely in the grid of sinusitis. I was also misled by her emphasis on allergies. Her dripping nose and choked voice led me astray. I might have missed her malaria because Occam’s razor was at the back of my mind- go with a single cause if it appears to explain all the data. I never thought that malaria could also co-exist with sinusitis. I didn’t think that her fevers were related to the deadly malarial parasites, and not to the inflamed sinuses. The diagnosis of malaria was serendipity – I was so reluctant to order a battery of blood tests to diagnose her fevers because I thought that they were not necessary. I had closed my differential diagnosis very early, and felt happy because I was able to explain the malady by a single diagnosis.

In his elegant book, How Doctors Think, Jerome Groopman explains succinctly why doctors often miss diagnosis. Doctors do not stumble because of their ignorance of medical facts; rather they miss diagnosis because they fall into three traps. Groopman gives a simple checklist of three questions before we make a premature – and often wrong diagnosis. First, we should ask, “What else could this be?” Second, “Could two things be going on to explain this problems?” Third, “Is there anything in the history, physical examination or lab tests that seems to be odds with the working diagnosis? The first question saves a physician from the cognitive trap; the second question forces a physician to look for other causes of the problem and the third question safeguards against confirmation bias.

I learned a lesson from this episode. Had I actively sought an alternate explanation for her fevers, malaria could have easily found a place in the differential diagnosis basket. After all, malaria is so common in Sevagram- my residents care for close to hundred severely ill malaria patients in a single season, and yet, I didn’t think of malaria when it mattered so much.

How do we avoid these errors? We need to approach each patient with a fresh eye, and unbiased mind. We should not shut the doors of differential diagnosis prematurely. Doing that, without ordering battery of just in case tests, is indeed a daunting challenge. Checklists have recently found their way in operating rooms worldwide – this 3-question checklist is as simple, and as effective in reducing medical errors caused by wrong diagnosis.